Age is an established risk factor for AD. Findings from the Baltimore Longitudinal Study indicate that the cumulative incidence of AD appears to double every five years from age 65, and is rarely seen in the younger population. In families with early onset AD, several genetic markers for AD have been discovered on Chromosomes 14 and 21.

In late onset AD, the most common type, patients with both familial and sporadic late onset AD have an unusually frequent occurrence of apolipoprotein E4 (ApoE4). The gene for apolipoprotein E is found on Chromosome 19. There are three alleles of Apo E (e2, e3, and e4). While e3 seems to be protective for AD, e4 (the gene that encodes for ApoE4) appears to increase the risk. People with Apo E4 are affected with the disease earlier in life. On average, people with two e4 alleles are likely to develop AD by age 68. People with one e4 allele will, on average, be affected by age 75. Fifteen percent of the overall population has one e4 allele and 1% of the population has two e4 alleles. It has been suggested that Apo E4 promotes the formation of beta amyloid and this protein has been found in association with plaques and tangles. Apo E3 promotes nerve growth in vitro, while Apo E4 inhibits it.

All patients with Down's syndrome who live past the age of 40 will develop AD. Because they have an extra Chromosome 21, these patients receive an extra dose of the abnormal amyloid precursor protein associated with AD.