男性雖然不像女性那么容易活到90歲，但那些步入90歲高齡的男性卻比女性患阿爾茨海默氏癥等老年癡呆癥的幾率要低很多。

Men are much less likely than women to live into their 90s, but those who do have a much lower chance of having Alzheimer's disease （阿爾茨海默氏癥）or another form of dementia, U.S. researchers said on Wednesday.

In a study involving 911 people age 90 and older, 28 percent of the men and 45 percent of the women had some type of dementia. Alzheimer's disease is the most common form of dementia in the elderly, and vascular dementia（癡呆） -- loss of brain function thanks to a series of small strokes -- is second.

While dementia rates remained stable for men through their 90s, they climbed in women, to 27 percent of the women aged 90 to 91 and 71 percent of women aged 98 to 99.

"This difference in prevalence -- basically how many people have the disease at any given time -- may be because women may live longer once they get diagnosed with dementia than men," said Maria Corrada of the University of California at Irvine, who led the study, said in a telephone interview.

Another possibility noted by the researchers was that relatively few men live to age 90 and beyond, and those who do may be hardy "survivors" with fewer risk factors for dementia.

Experts predict a worldwide surge in dementia cases in the coming decades as populations in many countries age.

"With this (age 90 and beyond) being the fastest-growing segment of the population in the United States and in many other countries too, we have to be prepared for the increasing number of people with this disorder that is very expensive to care for," Corrada said.

But much of the research on old-age dementia has not focused on nonagenarians （九十多歲的）and older. About three-quarters of people age 90 and up are women.

"Until this study, and a few others, there was very little data on what happens with rates of dementia in extreme old age, once you get beyond 85, up into the 90s and even higher," added Dr. David Hogan of the University of Calgary, who wrote an editorial in the journal Neurology accompanying the study.

Corrada's team studied a population of people 90 and above, most living in southern California. About 10 percent were at least 100.

The chances of having dementia doubled every five years in the 700 women after reaching age 90, but remained largely consistent in the 200 men.

As has been shown in other ages, women with more education were less likely to have dementia than less-educated women.

Previous studies had shown that dementia becomes more and more common starting when people are in their mid-60s through their 80s. For example, fewer than 2 percent of people ages 65 through 69 are estimated to have dementia, compared to 5 percent of those aged 75 to 79 and more than 20 percent of those 85 to 89.

In the new study, 41 percent of both the men and women age 90 and older were found to have dementia.

Dementia involves a loss of brain function, and symptoms of dementia include memory loss, mental disorientation and behavioral changes. It gets worse over time