Jean Piaget was a Swiss psychologist and biologist that studied the cognitive and intellectual development of children. Children are classified into one of four stages of development including the sensorimotor stage, the preoperational stage, the concrete operational stage, and the formal operational stage. Piaget believed that children only advance to the next stage when they have had a sufficient number of experiences within each stage.
From their birth to approximately two years of age, children are considered to be in the sensorimotor stage. During this stage, knowledge is gained through sensory experiences and the physical manipulation of objects. Object permanence is eventually attained in this stage. This is the ability to recognize that an object is still present even if it is unseen.
The preoperational stage, taking place between the ages of two and seven years, is the second stage of development. Children think egocentrically; it is difficult for them to see from another's perspective. While language and thinking skills are improving, children continue to think in concrete terms.
The third stage is the concrete operational stage, taking place from seven to eleven years of age. Egocentrism begins to fade and more logical thinking begins. However, this logical thinking still primarily applies to concrete objects. Children accomplish conservation, which means they understand that changing the form of an object does not change its amount or mass.
The fourth stage, the formal operational stage, begins at age twelve and progresses through adulthood. Abstract reasoning is an important characteristic of this stage. Hypothetical problems can be considered, and logical thought and problem-solving continues to improve.