Objective: The aim of the article is to analyze associations between sex, age, education, and social class and cognitive ability in late midlife and to evaluate differences in cognitive ability among the three Copenhagen Aging and Midlife Biobank (CAMB) cohorts. Method: The sample comprised 5,417 CAMB participants from three cohorts with scores on the Intelligenz-Struktur-Test 2000 R (I-S-T 2000 R). Results: Independent associations of cognitive ability with age, sex, education, and occupational social class were observed. Particularly, strong associations with cognitive ability were obtained for school education, and consistent sex differences were observed with higher cognitive ability in men. Differences in cognitive ability among the three cohorts were small and primarily reflected demographic differences. Discussion: Late-midlife cognitive ability is associated with a number of demographic factors, and demographic differences may contribute to individual differences in health and early aging. In analyses of cognitive ability, the three CAMB cohorts can be combined provided the relevant demographic variables are included as covariates.