Fortunately, people with intellectual disabilities are now living much longer lives in the community than when they lived in large institutions. Unfortunately, this also means that people with intellectual disabilities are developing various forms of dementia, including Alzheimer's disease, as they age - just like the general population.

Among people whose cause of intellectual disability is not Down syndrome, the incidence of Alzheimer's disease appears to be the same as in the general population. Among those with Down syndrome, however, Alzheimer's disease is occurring more often and at younger ages than in the general population. Symptoms in people with Down syndrome occur from 10 to 20 years earlier than in the general population, and among those 60 and older, 65% of people with Down syndrome have the disease.

Down syndrome, also known as Trisomy 21, is most often the result of a third (extra) chromosome on the 21st pair of chromosomes. Recently, genetic markers for Alzheimer's disease have been found on the 21st pair of chromosomes. Consequently, it is currently assumed that the higher incidence of Alzheimer's disease among people with Down syndrome is related to this finding, but further scientific research is needed to support this hypothesis.

For more information on the relationship between Down syndrome and Alzheimer's disease, see the following: