The timing of developmental changes, and whether they happen at all, vary profoundly from person to person. Many factors play a role, including the following.Age, Education, Gender
Abuse, Neglect, Trauma
Traumatic events, such as abuse, neglect, severe deprivation, and exposure to violence, take a costly toll. Young adults with a history of trauma are vulnerable to getting "stuck" developmentally, or to growing more slowly and/or unevenly than otherwise.
Race, Ethnicity, Sexual Identity
Anything that adds to challenges around identity can make the developmental tasks of young adulthood more difficult, including challenges associated with belonging to an oppressed, victimized, or stigmatized group within society.
Parents vary in the extent to which they provide opportunities for young adults to receive the support and the challenges that foster development. The role of parents in young adult development is only just beginning to be studied and appreciated, but it is clear that parents continue to have an important and evolving influence.
Any serious illness, especially mental illness, can create delays in healthy development. The high rates of depression and other mental illnesses among young adults in the U.S. are of particular concern.
Learning disabilities are a factor in development, as are differences between the learning style of the young adult and the educational approach of her or his learning environment.
Growing evidence points to the serious impact of chronic substance abuse on young adult development. Recent research is demonstrating ways in which alcohol and other drugs affect the growing brain, causing damage that may or may not be possible to repair.
Fascinating differences in young adult development across culture are just beginning to be explored, differences that allow some kinds of growth to occur earlier in some cultures than others. For example, young adults in cultures that emphasize interdependence and interconnectedness may adopt a more "multiplistic" view sooner than those in societies that emphasize individuality and independence.
Because of any of these circumstances, or a combination, some people may not make the kinds of shifts in complexity of thinking that typically occur in young adulthood. They struggle with the expectations and demands of modern life in part because they are handicapped by thinking capacities that are more typical in some ways of adolescence and younger ages.