INTRODUCTION: We examined the association between cognitive ability in young adulthood and dementia in Danish men, brothers, and male twins.

METHODS: In total, 666,986 men born between 1939 and 1959 were identified for dementia diagnosis in national registries from 1969 to 2016. The association between cognitive ability from draft board examination and dementia was examined using Cox regression.

RESULTS: During a 44-year follow-up, 6416 (0.96%) men developed dementia, 1760 (0.26%) and 970 (0.15%) of which were classified as Alzheimer's and vascular dementia, respectively. Low cognitive ability was associated with increased risk of dementia (hazard ratio [HR]per SD decrease 1.33 [95% confidence interval {CI} = 1.30-1.35]) with the strongest associations for vascular dementia (HRper SD decrease 1.47 [95% CI = 1.31-1.56]) and a weaker for Alzheimer's disease (HRper SD decrease 1.07 [95% CI = 1.03-1.13]). The intrabrother and twin analyses (taking shared family factors into account) showed attenuated risk estimates but with wide CIs.

DISCUSSION: Low early-life cognitive ability increases the risk of dementia before the age of 78 years. The association is partly explained by shared family factors.