FIGURE 12.3: Changes in two intellectual abilities over the decades in the Seattle Longitudinal Study: Notice that scores on a test demanding a heavier component of fluid skills (word fluency, which asks people to name as many words as they can starting with a letter such as A, within a time limit) decrease after the late forties; while those on a totally crystallized test (vocabulary) stay stable into the sixties. But in general, this landmark study shows intellectual abilities peak in the fifties and decline in old age.
Data from: Schaie, 1996.