The researchers reviewed an analysis of 911 people enrolled in one of the nation's largest studies of dementia and other health factors in the fastest-growing U.S. age demographic. Forty-five percent of the women had dementia, compared to 28 percent of the men, but researchers are not sure why dementia is more prevalent among the women the same age as men.

However, the results also showed that women with a higher education appeared to be as much as 45 percent less likely to have dementia compared to women with less education.

"Our findings show that more will need to be done to provide adequate resources to care for the increasing number of very old people with dementia," study corresponding author Maria Corrada said in a release.

The frequency of dementia increases with age from less than 2 percent for the 65- to 69-year-olds, to 5 percent for the 75- to 79-year-olds and to more than 20 percent for the 85 to 89-year-olds.

The study appears in the online issue of Neurology.