<P> Fluid intelligence , like reaction time , typically peaks in young adulthood and then steadily declines . This decline may be related to local atrophy of the brain in the right cerebellum . Other researchers have suggested that a lack of practice , along with age - related changes in the brain may contribute to the decline . Crystallized intelligence typically increases gradually , stays relatively stable across most of adulthood , and then begins to decline after age 65 . The exact peak age of cognitive skills remains elusive , it depends on the skill measurement as well as on the survey design . Cross-sectional data shows typically an earlier onset of cognitive decline in comparison with longitudinal data . The former may be confounded due to cohort effects while the latter may be biased due to prior test experiences . </P>
what happens to the crystallized intelligence as one goes from young adulthood to middle adulthood
stays relatively stable