Patients with Alzheimer's and dementia are seen during a therapy session inside the Alzheimer foundation in Mexico City April 19, 2012. ( Edgard Garrido / Reuters)

The sons and daughters of people who live very long lives tend to get the symptoms of Alzheimer's disease later than others, but they're not immune from the memory-robbing disease, according to a new study.

Based on comparisons of people in their 90s, their spouses, siblings, children and their children's spouses, researchers found that the offspring of people with exceptional longevity were about 40 percent less likely than peers to be cognitively impaired between ages 65 and 79.

"It's not necessarily that these individuals never become cognitively impaired, but what it seems like is that there is a delayed onset of cognitive impairment," said Stephanie Cosentino, of the Columbia University Medical Center in New York.

By the time the older generation of study volunteers were in their 90s, however, their risk of being cognitively impaired was fairly high.

So Cosentino's team projects that the kids of these long-lived individuals will have the same risk level as their parents if they enjoy similar longevity - that is, they'll no longer be protected.

Loosely defined, longevity means living beyond the average age of death among peers. In the U.S. today, for instance, a 65 year old man can expect to live to age 83, on average, and a woman to age 85.

As life expectancies continue to rise, few have investigated whether that means people live to those old ages cognitively "intact," Cosentino and her colleagues write in JAMA Neurology.

Alzheimer's disease is diagnosed in about 5,000 Americans each year. It's the most common form of dementia, affecting more than 5 million Americans, according to the National Institute on Aging.

Both longevity and dementia risk have some degree of heritability.

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Family longevity could lessen instances of dementia