There is currently little consensus regarding what must be explained by theories of cognitive aging. In the present article, four empirical generalizations that seem to imply certain constraints in theorizing are identified. These generalizations, and their possible implications or constraints, are that (1) age-related differences are found in a wide range of cognitive variables, implying that either a large number of specific factors or a small number of general factors must be contributing to the agerelated differences; (2) the age-related influences on different cognitive variables are not independent, and unique age-related influences appear to be few in number and small in magnitude, implying that some fairly general factors need to be postulated to account for the shared age-related influences; (3) only a small proportion of distinct age-related variance occurs late in practice and at long presentation durations, implying that adequate explanations must include factors operating when the individuals are just beginning to perform the task and when the stimuli can first be registered; and (4) measures of how quickly very simple cognitive tasks can be performed share considerable age-related variance with many cognitive variables, implying that factors related to simple processing efficiency need to be incorporated into the explanations.Research concerned with the relations between adult age and cognitive performance has been increasing dramatically over the last several decades. Despite the rapid expansion of research, however, there is still little consensus regarding the reasons for the negative relations that are typically reported between chronological age and the measures of memory, reasoning, and spatial ability sometimes referred to as comprising Type A (Hebb, 1942) or fluid (Horn & Cattell, 1963) cognition. The primary purpose of this article is to describe several sets ofempirical results that appear to place important constraints on theoretical explanations of age-related cognitive decline phenomena.It is helpful to begin by describing the broad phenomenon in need ofexplanation by theories ofcognitive aging. Consider the distribution of scores on a cognitive test, such as immediate free recall of a list of unrelated words. In a recent experiment (Salthouse, 1993b), a total of305 adults between 19 and 84 years of age attempted to remember two 12-word lists (presented auditorily at a rate of 1 word every 2 sec), and their average scores across the two lists ranged from near zero to perfect (top panel of Figure I). Because the sample included people of different ages, the distribution can be disaggregated by age. That is, the individuals can be ordered by their ages, and then the scores plotted by age (as in the bottom panel of Figure 1). This type of disaggregation typically results in considerable variability at each age, but with an average This research was supported by National Institute on Aging Grant R37 AG6826 to T.A.S. I would like to thank three reviewers for their constructive comments on a...