1 Alzheimer's disease (AD) is the most common cause of dementia in people over age 65 with cause possibly related to amyloid plaques and neurofibrillary tangles.2  Almost all brain functions, including memory, movement, language, judgment, behavior, and abstract thinking, are eventually affected. Huntington's disease is a heredity disorder caused by a faulty gene. 2  Children of those with the disorder have a 50% chance of getting the disease. 3  Symptoms begin in those 30 to 40 years of age, beginning with personality changes such as anxiety and depression.