An age-related, non-reversible brain disorder that develops over a period of years. Initially, people experience memory loss and confusion, which may be mistaken for the kinds of memory changes that are sometimes associated with normal aging. The symptoms gradually lead to behaviour and personality changes, a decline in cognitive abilities such as decision-making and language skills and problems recognizing family and friends; and ultimately to a severe loss of mental function. Alzheimer's disease is one of a group of disorders called dementias that are characterized by cognitive and behavioural problems. It is the most common cause of dementia among people age 65 and older.