Cohort differences in trajectories of cognitive aging.

To separate age and cohort effects on decline in normal cognitive aging, we applied growth curve models to longitudinal data from the Swedish Adoption/Twin Study of Aging. Data from up to five measurement waves covering a 16-year period were available from 806 participants (age 50 to 88 at the first wave). We divided the sample into two cohorts by birth year: 1900-1925 and 1926-1948. We generated components to tap four cognitive domains: verbal and spatial ability, memory, and speed. We tested cohort differences by using two growth models: quadratic and two linear slopes. Results indicated significant cohort differences in average performance at age 67.5 for all components except speed. When we compared linear slopes during the same age range (age 62-78), we found no cohort differences. Trajectories of change with age in these four domains were fundamentally the same in middle-old age for individuals born during the first half of the 20th century.