Laguna Woods Village, a sprawling retirement community of 20,000 south of Los Angeles, is at the center of the world’s largest decades-long study of health and mental acuity in the elderly. Begun by University of Southern California researchers in 1981 and called the 90+ Study, it has included more than 14,000 people aged 65 and older, and more than 1,000 aged 90 or older.

Such studies can take years to bear fruit, and the results of this study are starting to alter the way scientists understand the aging brain. The evidence suggests that people who spend long stretches of their days, three hours and more, engrossed in some mental activities like cards may be at reduced risk of developing dementia. Researchers are trying to tease apart cause from effect: Are they active because they are sharp, or sharp because they are active?

The researchers have also demonstrated that the percentage of people with dementia after 90 does not plateau or taper off, as some experts had suspected. It continues to increase, so that for the one in 600 people who make it to 95, nearly 40 percent of the men and 60 percent of the women qualify for a diagnosis of dementia.

At the same time, findings from this and other continuing studies of the very old have provided hints that some genes may help people remain lucid even with brains that show all the biological ravages of Alzheimer’s disease. In the 90+ Study here, now a joint project run by U.S.C. and the University of California, Irvine, researchers regularly run genetic tests, test residents’ memory, track their activities, take blood samples, and in some cases do postmortem analyses of their brains. Researchers at Irvine maintain a brain bank of more than 100 specimens.

To move into the gated Laguna Woods Village, a tidy array of bungalows and condominiums that blends easily into southern Orange County, people must meet several requirements, one of which is that they do not need full-time care. Their minds are sharp when they arrive, whether they are 65 or 95.