5% of people over the age of 65 have dementia, and 20% of over 80s. Alzheimer’s Disease is the big mahoona, making up to 70% of dementias, and is detailed below. However, there are some other important types:
Progressive dementia, usually starting with short term memory loss, leading to decline in other cognitive functions and eventualy death.
Around 65% of dementias are Alzheimer's. It is slightly more common in women, and rarely hits under the age of 60.
It also develops in 50% of those with Down's Syndrome who live over the age of 40.