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. KEYWORDS—aging; cognition; reasoning; memory; speed The phenomenon of cognitive aging has been noticed almost as long as the phenomenon of physical aging, but it is still not well understood. This is unfortunate because cognitive functioning can affect one’s quality of life, and even the ability to live independently. Furthermore, cognitive functioning in early adulthood may be related to the development of pathologies such as Alzheimer’s disease in later adulthood. One way to conceptualize understanding is that it is equivalent to knowing answers to the questions of what, when, why, where, and how. In this article, I summarize some of the progress that has been achieved in describing the phenomenon of cognitive aging in terms of the questions of what and when. Although not much is yet known about why (what is ultimately responsible), where (in the nervous system), and how (via what mechanisms) age-related cognitive changes occur, a key assumption of my research is that answering these other questions will be easier as the characterization of what and when becomes more