The book analyzes the Seattle Longitudinal Study, which Professor Schaie began as a graudate student in the 1950s. The study has been impressive in its methodological sophistication, inclusion of a broad array of variables related to intellectual development, and attention to individual differences in intellectual aging.

Up to the 1950s, studies of intelligence focused on children and college students, and the popular notion was that intelligence peaked at age 16 and declined in older adults in a uniform way. In his early work, Professor Schaie discovered that that dominant concept of intelligence was simplistic and that there are many variations in terms of when intelligence peaks and declines, as well as many different factors that affect a person's intelligence.

Important practical questions are raised, such as: At what age do developmental peaks occur, and what are the generational differences and within-generation age changes? How do you establish sufficient competence for independent living?