Definition of Dementia

Dementia is a syndrome due to disease of the brain, usually chronic, characterized by a progressive, global deterioration in intellect including memory, learning, orientation, language, comprehension and judgment. It is one of the major causes of disability in later life.
Alzheimer’s disease is the most common type of dementia. Other types include vascular dementia, dementia with Lewy bodies and frontotemporal dementia. The boundaries between the types are not clear, and a mixture of more than one type is common. Dementia mainly affects older people, although there is  a growing awareness of cases that start before the age of 65. After age 65, the likelihood of developing dementia roughly doubles every five years.
Dementia mainly affects older people, however, between 2% and 10% of all cases start before the age of 65. After this, the prevalence doubles with every five year increment in age.