June 24 (HealthDay News) -- To stave off the mental decline associated with old age, engage in intellectually challenging activities, maintain a positive outlook and keep up your social life.

Those are the findings of what researchers say is the largest-ever review of studies on aging and the brain.

The review, which spanned three decades and covered more than 400 studies, found that remaining physically, mentally and socially active has a substantial impact on whether older adults experience declines in memory and cognition, which includes the ability to learn and solve problems.

"How people spend their lives does really have an impact on how they age cognitively," said study co-author Robert S. Wilson, a professor of neurological and behavioral sciences at Rush University Medical Center in Chicago. "The observational studies suggest people who are more intellectually active, socially integrated, physically active and who are relatively free from negative emotions like depression and anxiety all seem to be associated with aging better cognitively."

As the U.S. population ages, being able to keep mental decline at bay for even a little longer could have significant public health implications.

A hundred years ago, only about 4 percent of the U.S. population was older than 65. In 2000, that group reached more than 12 percent. By 2030, an estimated 20 percent will be older than 65.

Along with this, the prevalence of Alzheimer's disease is expected to quadruple over the next 40 years, Wilson said.

"There is going to be a huge burden of old people who are cognitively impaired," Wilson said. "If we can develop strategies that delay the onset of the disease by six months or a year or two, we can substantially reduce the human suffering and the cost of caring for them."

The study, which will appear in Psychological Science in the Public Interest, was released June 24 in Washington, D.C.

It identified several aspects of a person's lifestyle that can have a major impact on mental function: exercise, staying socially integrated, participating in mentally stimulating activities and maintaining an optimistic, agreeable, goal-oriented attitude.

Though exercise had a powerful impact on mental function, the type of exercise made a difference.

In studies that asked older people how physically active they were, those who reported doing the most exercise had somewhat better mental functioning than those who were more sedentary, but the effect wasn't dramatic.

However, people who took part in studies that put them on a regular aerobic exercise program saw substantial gains in mental functioning.

Among older adults, even those who do relatively more exercise than their peers probably aren't doing all that much, Wilson said.

"Left to their own devices, most older people in this country don't exercise all that much at all," Wilson said. "Any exercise is good, but actually doing a regular program of aerobic exercise is better."

Walking, the most commonly cited exercise, can be part of an aerobic exercise program, but the pace must be fast enough to raise the heart rate.

Recent research has looked at whether specific products or programs, such as video games, improve cognitive functioning in older people. Although there is nothing on the market that is scientifically proven to increase memory and thinking skills, Wilson said, he thinks such products could be available soon.

But you don't have to go out and buy something to engage in intellectually stimulating activities, said Dr. Gary Kennedy, director of geriatric psychiatry at Montefiore Medical Center in New York City.

Doing a crossword puzzle, playing chess or learning a language can fit the bill. Research shows that taking part in activities that are novel, challenging and in which you are personally invested can have a lasting impact on mental functioning.

"The reality is, as we age, cognitive processes slow down," Kennedy said. "It may take you longer, and you may have to practice longer to master something new, but hopefully you have health and the time to actually do that."

Though cognitive decline was once seen as an inescapable part of aging, public perception is beginning to change, and recent studies back that up.

"Most people's brains are under assault in old age, and the lifestyle stuff doesn't appear to stop that pathology," Wilson said. "But lifestyle does appear to help your brain tolerate that pathology. It helps you get more out of what you have left and to adapt to the changes in your brain, and it appears to make a big difference."

In an accompanying editorial, Jonathan W. King and Richard Suzman, from the U.S. National Institute of Aging, said that the study's findings overall paint a "fairly optimistic" picture.

"It could well be possible to design interventions that, when combined with appropriate lifestyle changes, could possibly at least slow the rate of cognitive decline," they wrote.

SOURCES: Robert S. Wilson, Ph.D., professor, neurological and behavioral sciences, Rush University Medical Center, Chicago; Gary Kennedy, M.D., director, division of geriatric psychiatry, Montefiore Medical Center, New York City; Psychological Science in the Public Interest