Erik Erikson in the context of "Young adult"

Adult development encompasses the changes that occur in biological and psychological domains of human life from the end of adolescence until the end of one's life. Changes occur at the cellular level and are partially explained by biological theories of adult development and aging. Biological changes influence psychological and interpersonal/social developmental changes, which are often described by stage theories of human development. Stage theories typically focus on "age-appropriate" developmental tasks to be achieved at each stage. Erik Erikson and Carl Jung proposed stage theories of human development that encompass the entire life span, and emphasized the potential for positive change very late in life.

The concept of adulthood has legal and socio-cultural definitions. The legal definition of an adult is a person who is fully grown or developed. This is referred to as the age of majority, which is age 18 in most cultures, although there is a variation from 15 to 21. The typical perception of adulthood is that it starts at age 18, 21, 25 or beyond. Middle-aged adulthood, starts at about age 40, followed by old age/late adulthood around age 65. The socio-cultural definition of being an adult is based on what a culture normatively views as being the required criteria for adulthood, which in turn, influences the lives of individuals within that culture. This may or may not coincide with the legal definition. Current views on adult development in late life focus on the concept of successful aging, defined as "...low probability of disease and disease-related disability, high cognitive and physical functional capacity, and active engagement with life."

A character structure is a system of secondary traits manifested in the specific ways that an individual relates and reacts to others, to various kinds of stimuli, and to the environment. A child whose nurture and/or education cause them to have conflict between legitimate feelings, living in an illogical environment and interacting with adults who do not take the long-term interests of the child to heart will be more likely to form these secondary traits. In this manner the child blocks the unwanted emotional reaction that would have normally occurred. Although this may serve the child well while in that dysfunctional environment, it may also cause the child to react in inappropriate ways, by developing alternate ways in which the energy compulsively surfaces, ways damaging to his or her own interests, when interacting with people in a completely independent environment. Major trauma that occurs later in life, even in adulthood, can sometimes have a profound effect on character. See post-traumatic stress disorder. However, character may also develop in a positive way according to how the individual meets the psychosocial challenges of the life cycle (Erikson).