In 1906, Dr. Alzheimer noticed changes in the brain of a woman who had died of an unusual mental illness. Within the brain tissue, he found abnormal clumps and tangled bundles of fibers, which are now considered classic signs of Alzheimer's disease. The disease usually begins after age 60, and the risk of developing this condition goes up with age. While people may get Alzheimer's disease at a younger age, it is much less common.