What is lifespan development?
Lifespan development is a term describing the changes and stages of life humans go through as we age. The term also accounts for any "developments" in mental or physical processes which contribute to the next stage of life. For example, infancy is a stage of life, during which humans gain basic motor and social skills. Without basic skills in speech ("baby talk,") mobility (crawling or walking,) and self-care (feeding oneself,) the next phase of life is very difficult. Later in life, development or lack thereof of more fine-tuned mental and physical processes can really "make or break" certain aspects of someone's life.
Let's turn to the famous example of Genie, who was raised in extreme isolation in her family's home. Even though she was raised through interaction with her parents, she was considered a feral child. Her father suffered mental instability and was abusive to Genie, her mother, and her brother. He forbade anyone from speaking around Genie or from releasing her from a potty-chair, to which she was strapped during the day. Because of her lack of social engagement or freedom to develop motor skills, Genie suffered some challenges later in life. After being freed at thirteen, she required near constant care and therapy as she could not speak, walk, or perform basic self-care actions. Genie missed out on a lot of learning and development that typically takes place within the first several years of life, and because of this, she didn't really have a foundation to progress beyond early childhood capabilities.
Studying lifespan development can help us evaluate various milestones for human development and how their presence or absence impacts later developments. As a branch of psychology, lifespan development primarily focuses on the mental and emotional capacities of people at various ages.