Older age does not cause Alzheimer’s, but it is the most important known risk factor for the disease. The number of people with Alzheimer’s disease doubles about every 5 years beyond age 65. About one-third of all people age 85 and older may have Alzheimer's disease.
Scientists are learning how age-related changes in the brain may harm neurons and affect other types of brain cells to contribute to Alzheimer’s damage. These age-related changes include atrophy (shrinking) of certain parts of the brain, inflammation, vascular damage, production of unstable molecules called free radicals, and breakdown of energy production within cells.
However, age is only one risk factor for Alzheimer’s disease. Many people live into their 90s and beyond without ever developing dementia.