What and when of cognitive aging

Adult age differences have been documented on a wide variety of cognitive variables, but the reasons for these differences are still poorly understood. In this article, I describe several findings that will need to be incorporated into eventual explanations of the phenomenon of cognitive aging. Despite common assumptions to the contrary, age-related declines in measures of cognitive functioning (a) are relatively large, (b) begin in early adulthood, (c) are evident in several different types of cognitive abilities, and (d) are not always accompanied by increases in between-person variability.