The study also found that Alzheimer's disease accounted for almost 70 percent of all dementia, while stroke-related dementia, called vascular dementia, accounted for around 17 percent of all the cases.
With the onset of old age, Alzheimer's contributed to a significantly higher proportion of dementia cases. In the 90-plus age group, it comprised nearly 80 percent of all dementia cases.
Participants were visited by trained nurses and neuropsychology technicians at their homes around the country. They were examined using a diagnostic test similar to one used in most memory evaluations.
"These conditions affect millions of older Americans and touch nearly every family in some way, and the situation is only going to get worse as the population ages," Duke University lead author Brenda Plassman said.
Researchers did not find any significant differences between males and females. They did, however, find that the higher the education level, the lower the risk of dementia.
The study's findings have been published in Neuroepidemiology.
