Alzheimer's disease is an irreversible, progressive brain disorder that gradually destroys a person’s memory, thinking ability, and other important mental functions. It is the most common cause of dementia in older adults. People often develop the condition around their mid-60s. With Alzheimer's disease, the brain cells degenerate and die. This causes a slow reduction in memory and mental function. Alzheimer's disease is currently listed as the sixth leading cause of death in the United States. It is estimated that about five million people in the United States have Alzheimer's disease.