What is aging?

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A series of time-dependent anatomical, physiological, and psychological changes that diminish physiological reserve and functional capacity and that produce emotional transformations. Some changes can also signal positive processes of maturation.
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Physical and Psychological Factors

Aging takes place over the course of life, and the rate of change varies between individuals and groups. Differences in aging are genetically determined in part, but there is also a substantial environmental component. These environmental factors can include nutrition, lifestyle choices, and toxins in the environment. Primary aging relates to the genetic components of aging, while secondary aging focuses on environmental factors.

Aging is influenced by a number of normative age-graded factors that typically take place at a particular chronological time. These factors can be biological, as reflected in puberty and menopause, as well as environmental, having an impact on the socialization of the individual, such as the changes needed to enter school or to assume a work role. Normative history-graded factors refer to events that are shared by a society and have an impact, either positive or negative, on the aging process. The Great Depression in the 1930s, World War II, and the Vietnam War are examples of normative history-graded events. When a major event has an impact on a single generation in a society, it is known as a cohort effect. When an event affects the entire population, it is termed a period effect. The Great Depression is considered to be a period effect, while the Vietnam War is labeled a cohort effect. Normative age-graded factors are considered to be most important during childhood and in old age, while normative history-graded events are considered to have their greatest impact on aging in the early and middle adulthood periods of life. Nonnormative factors recognize that each individual experiences unique factors that influence the personal aging process. A natural disaster, a divorce, and winning the lottery are considered to be nonnormative factors.

A multitude of age-related changes are typically benign and permit an individual to function, perform daily functions, and remain active in society. With increasing age, however, a number of biological changes lead to a decline in efficiency of function and performance. Some general signs of bodily aging include decreases in physical stature and loss of bone mineral density. A quantitative loss of cells in the body results in a decrease in overall muscle mass, perhaps as high as 80 percent. A loss of fat beneath the skin results in an increased sensitivity to temperature extremes. In general, the skin of the aging person atrophies with shrinking of the sweat and sebaceous glands, leading to dry and itchy skin. A decline in blood vessels causes a slowing of healing, and a loss of skin elasticity occurs with the breakdown of elastin.

Age-related changes in the cardiovascular system include narrowing of the blood vessels due to the thickening of the endothelial lining and a decline in smooth muscle mass. The blood vessels become rigid and contribute to a gradual elevation of blood pressure. Changes in the heart include thickening of the myocardium, a reduction in size of the ventricular cavities, and a decrease in volume of blood pumped per contraction. Heart rate may slow with age, as the cells in the sinus node can decline up to 90 percent. Consequently, elderly people may show limited heart rate increases when experiencing stress or trying to increase activity level. Changes in the respiratory system show a number of functional declines. Smooth muscle in the bronchi, diaphragm, and chest wall becomes weakened as a result of an increase of collagen deposits, contributing to a diminished work capacity in late life. Reductions in the lung surface area, decreased maximum ventilatory volume of the lungs, and limitations on maximum oxygen utilization can produce senile emphysema. This age-related condition limits the amount of exercise and energy that an older person can expend at any given time. By age sixty-five, an individual cannot fully expand the chest when seated. Secondary aging factors can contribute significantly to the diminished function of the respiratory system, as pollutants in the environment and smoking have been found to exaggerate these changes.

Musculoskeletal changes caused by aging include a loss of muscle mass, with striated musculature diminishing by approximately 50 percent by age eighty. As these cells are lost, they are replaced by fat cells. With age, the mineralization of bone declines for both men and women, but women experience osteoporosis at an accelerated rate after menopause. Both older men and older women are at increasing risk of fractures with age, but women experience this enhanced risk ten years sooner than do men.

In the nervous system, the brain shows a number of normative changes with age. In general, the brain decreases in size, primarily due to loss of mass in the white matter. Gray matter loss contributes significantly to this decrease after age seventy-five. Brain weight also diminishes beginning at age forty. The somatosensory system provides information from nerve endings located in the skin, joints, and muscles. As people age, the ability of the somatosensory system to detect stimuli in the environment and to provide accurate sensory information declines. Older adults have difficulty perceiving joint movement and position; this decline places them at an increased risk for falls.

Significant changes take place in the gastrointestinal system, with impaired gastric acid production in the stomach, slowing of the transit time in the large intestine, and decreased absorptive surface areas in the small intestine. The liver decreases in size and weight and shows a reduction in function. Other significant age-related changes to biological function include decreased renal blood flow; impaired function of the immune system; diminished levels of estrogen, progesterone, and testosterone; and decline in thyroid hormone levels.

Most noticeable to many older adults are the age-related changes to the sensory systems, as visual and auditory losses can routinely have an adverse effect on the performance of daily activities. With age, the lens and cornea of the eye thicken and show a yellowing. The lens is less flexible and increases in opaqueness, making focus more difficult. Production of the aqueous humor, the clear liquid that fills the anterior chamber of the eye, is reduced, leading to the increase in intraocular pressure associated with glaucoma. The ability to track moving objects declines as smooth pursuit eye movements show decrements. With advancing age, the auditory system shows a diminished capacity for sensitivity to higher frequencies, a condition known as presbycusis. According to the National Center for Health Statistics, 25 to 40 percent of adults over age sixty-five report concerns about a decline in speech perception. The problem is worsened in noisy environments. Taste and smell decrease in sensitivity with age to a degree that eating experiences become less pleasurable.

Although one unifying biological theory of aging does not exist, the National Institute on Aging has proposed two categories of aging theories: program theories and error theories. The program theories suggest that aging is due to a switching on and off of specific genes. When defects develop in the switching process, people experience the biological changes and functional declines associated with primary aging. The genetic nature of aging is shown in the maximal life spans that are found in all species. These species-specific differences in life span suggest an underlying genetic basis for aging. Since aging involves a complex array of normative age-related changes, it is expected that many interacting genes are involved in the overall process of growing old. One example of a program theory is the deliberate biological programming theory. This theory evolved from the research of Leonard Hayflick, who identified a limit for cell duplication. The theory holds that within a normal cell is a store of memory that dictates its life expectancy.

In contrast, the error theory approach maintains that aging is due in part to the wear and tear process that takes place over time. This approach posits that important components in the biological makeup of individuals wear out and cannot be repaired or replaced. Toxins in the environment and free radicals may contribute to the reduction in function of various systems as a result of wear and tear. Free radicals are chemical molecules with an odd number of electrons, which makes them highly reactive with other chemical compounds. Free radicals are produced through normal metabolic processes as well as by exposure to ionizing radiation, ozone, and chemical toxins. Research has shown that free radicals are associated with deoxyribonucleic acid (DNA) damage, cross-linkage with collagen to alter the characteristics of connective tissue, and cancer.

Psychological theories of aging usually are concerned with explaining the differences in behavior, changes in behavior, and patterns of action shown among persons of various ages. The psychological theories can be divided into three major categories: stability template, orderly change, and random change. The stability template approach emphasizes that the primary factors associated with development take place during infancy and childhood. It is during the early years of life that one’s personality is formed, and this provides a template for all future behaviors. As the aging individual faces new situations or challenges from the environment, behavior can be predicted based on the outcomes of early childhood experiences. From the stability template perspective, the older adult often demonstrates regressive behaviors, acting in childlike fashion, when normative age-related biological changes produce excessive stress. The resulting anxiety causes the older person to seek behaviors that were rewarding or comforting in the past. The theories of Sigmund Freud are an example of the stability template approach and promote the idea that the essentials of personality development are concluded at age five or six.

The orderly change theories suggest that aging takes place in a predictable pattern throughout the life span. This pattern is defined by specific stages that accompany the progression through life. Orderly change theorists believe that each stage of life—adolescence, young adulthood, middle age, and late life—presents specific challenges or tasks that must be completed in order to experience successful aging and avoid negative emotions such as depression. The eight stages of development described by Erik Erikson are an example of an orderly change theory.

The random change perspective emphasizes the nonnormative factors of aging and suggests that development takes place as a result of a variety of events that may or may not have an impact on individuals at different points in life. This approach is highlighted by the work of Paul B. Baltes, who concludes that a wide variation occurs in the behavior of people as they age, which in turn is dependent upon each individual’s particular circumstances.

In addition to general developmental theories, the psychological factors of aging focus on the changes in mental processes. Psychology differentiates between fluid intelligence and crystallized intelligence. Fluid intelligence is considered to be the basic abilities and cognitive skills found in a person. It reflects on the quality of one’s brain and how quickly information is perceived and processed, how well associations and patterns are recognized, and the efficiency of memory function. This is analogous to a computer’s hardware structure. Crystallized intelligence refers to culturally based and acquired cognitive functions. Development is dependent on experience with the world and the formal education system. The computer analogy would be its software. Development of cognitive ability is usually viewed as a stage phenomenon that begins during infancy when the child interacts with the surrounding environment through his or her reflexes and continues throughout life. Exposure to an enriched environment facilitates the development of crystallized intelligence. As a person reaches old age, tests of intelligence requiring speed or reaction time typically show a decline in ability. The developmental theory of Jean Piaget is an example of a well-known and highly researched approach to explain cognitive development from infancy onward.

Another significant approach to explain aging has been through the formation of social theories focusing on activity level throughout the life span. Disengagement theory suggests that aging inevitably requires a process of separating or disengaging oneself from physical, social, and psychological efforts. This disengagement is needed in response to the biological changes associated with old age. Activity theory was designed as an alternative view of aging and holds that normative aging requires remaining active throughout life. High activity levels are seen as positively related to good health, longevity, and life satisfaction. Most recently, continuity theory has been proposed as a compromise position between disengagement and activity theories. The continuity approach promotes the notion that older adults must maintain behavior according to the pattern of life that had been established before old age.

Disorders and Effects

Hutchinson-Gilford progeria syndrome and Werner’s syndrome are two forms of accelerated aging called progeria. The person with Hutchinson-Gilford syndrome experiences dwarfism and pseudosenility. Individuals with this genetic form of accelerated aging appear like very old and small humans. They usually die in their teenage years of coronary heart disease. Werner’s syndrome, also caused by genetic factors, affects persons in their late twenties and thirties and produces a shortened life span. The afflicted person ages very rapidly and develops a pinched facial expression, cataracts, diabetes, hypogonadism, a beak-like nose, prominent teeth, and a recessive chin.

Dementia is a fairly common condition that increases in frequency with age. It involves deficits in two or more areas of cognition that have a negative impact on daily functioning. Dementia produces impairment of memory and orientation and disruption in the ability to plan, organize, sequence, and make decisions. The majority of persons in old age who experience dementia have Alzheimer’s disease. That Alzheimer's Association estimated that in 2012, over five million Americans were living with the disease and that the number of cases of Alzheimer’s disease would increase to nine million by the year 2025. The risk for Alzheimer’s disease increases by approximately 1 percent with each year of life after age sixty-five. It is a progressive deteriorating disorder that attacks memory first before progressing through wandering, aggressiveness, and confusion of time and space to an eventual loss of self-awareness and a total inability to assume self-care. The characteristic brain pathology of Alzheimer’s disease includes cell loss, neurofibrillary tangles, and senile plaques found throughout the neocortex of the brain but concentrated in the hippocampus, frontal, parietal, and temporal lobes. Persons with Alzheimer’s disease can experience loss of approximately 40 percent of their brain mass. Four different chromosomes have been identified that heighten risk for the development of Alzheimer’s disease. Currently, there is no cure for the disorder and treatment is primarily supportive. Cholinesterase inhibitors and memantine, which improve the naturally occurring neural transmitter acetylcholine in the brain and have been shown to delay the progression of the disease, have been the medications of choice in treatment. Vascular dementia is caused by underlying cerebrovascular disease and has a more abrupt onset compared to Alzheimer’s disease.

Many of the age-related changes experienced among older adults have been suggested as contributing to the development of late-life depression. Depression is the most common emotional disturbance found in the elderly population. The prevalence of depression is highest among those older adults experiencing some medical illness or functional impairment. Suicide rates are high among older adults and highest among white men over the age of eighty-five. Many older adults experience “chronic suicide,” a slow steady decline in health and function caused by personal neglect of one’s needs.

One of the major effects of age-related changes is the need to compensate for functional declines. For many older adults to live independently, often requires some degree of home care. Impairments are measured in terms of daily living activities, which include eating, bathing, toileting, and moving in and out of chairs and bed. Remaining in the home can take on a significant level of psychological importance, since it is part of an older person’s identity and helps to maintain a sense of autonomy and control in one’s life.

Perspective and Prospects

Historically, the ancient Greeks provided some of the first writings on aging in discussing how old age brings increased anxiety about death. During the Roman era, Cicero suggested ways for elderly people to make themselves useful in advisory and administrative roles. He emphasized the importance of a developed mind and enhanced character as compensations for physical decline. The first manual to describe the problems associated with aging was published in the fifteenth century. Overall, however, historical writers had little to say about the positive attributes of aging.

Many common beliefs about aging have often resulted in oversimplified and biased stereotypes about older adults. These stereotypes may portray elderly persons as uninterested, weak, unattractive, undesirable, rigid, incapable of sexual activity, conservative, and lacking in intellectual acuity. This perspective encourages discrimination against older persons and is termed ageism. Senility is not a medical term but is commonly used in US society. It implies that old people lose the intellectual capacity to make intelligent decisions. As the number of older adults has increased and the importance of scientific investigations of aging has been expanded, positive counterpoints to aging have emerged. A new realistic image of older adults portrays the healthy elderly population as resourceful, optimistic, intelligent, flexible, and sexual beings. The study of aging has increased in importance as the geriatric population continues to grow in number and proportion of the population. According to census conducted in 2010 by the US Census Bureau, the sixty-five and over population in the United States is growing at a faster rater than is the general population. People are living longer as a result of the control of infectious diseases and improvements in health care, sanitation, and nutrition. The segment of the population aged sixty-five and older grew from 5 percent of the population in 1900 to 13 percent in 2010, and it is projected to increase to 22.9 percent by the year 2050. In 2000, there were approximately 35 million Americans over the age of sixty-five; in 2010, there were 40.3 million over the age of sixty-five; and this number is projected to increase to 70 million by 2030. Life expectancy at birth increased from forty-nine years in 1900 to approximately eighty-one years for women and seventy-six years for men in 2010. (Women tend to outlive men because of higher male mortality caused by heart disease, lung cancer, and emphysema.)

Despite the increases in the older population, aging is a relatively new topic of study. Initially, the pioneers in the fields of development focused on describing, explaining, and understanding the infancy and childhood periods of life. Researchers next moved to the study of adolescence, and they considered old age as a period of deterioration with a focus on chronic disease. Gerontology, the formal study of the phenomena of aging from maturity to old age, began producing significant research only in the 1950s. Geriatrics, the medical treatment of elderly patients, was proposed as an academic discipline in 1987 by the National Institute of Medicine.


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Cohen, Gene D. The Mature Mind: The Positive Power of the Aging Brain. New York: Basic Books, 2006.

Eaton, William W., ed. Medical and Psychiatric Comorbidity over the Course of Life. Washington, D.C.: American Psychiatric Publishing, 2006.

Hill, Robert D. Positive Aging: A Guide for Mental Health Professionals and Consumers. New York: W. W. Norton, 2005.

Kunkel, Suzanne R., J. Scott Brown, and Frank Whittington, eds. Global Aging: Comparative Perspectives on Aging and the Life Course. New York: Springer Publishing, 2014.

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