In this cohort study including 164 participants with symptomatic AD and a parental history of dementia, the factors of parental inheritance, more years of education, and retrospective determination of AAO were associated with an earlier-than-expected AAO of AD; parental history of early-onset dementia, APOE ε4 allele status, and hypertension were associated with a later-than-expected AAO of AD. This evidence concerns the gene APOE and dementia.