Alzheimer's disease is the leading cause of decline in mental abilities (dementia).

Alzheimer's usually affects aging adults. The disease is uncommon in people younger than 65 and is rare before the age of 45. Close to half of the people with the disease are between the ages of 75 and 85.

A rare, inherited form of the disease may affect some people as early as their 40s. This is called autosomal dominant Alzheimer's disease or early-onset Alzheimer's.