The 16th century Spanish explorer Ponce de Leon is famous for his fruitless search for the fountain of youth. But today the percentage of Americans age 65 and older has more than tripled, now representing more than 13 percent of the overall population.

Not only are more people living into the second 50 years of life, 70,000 centenarians have entered their third 50 year cycle. The U.S. Census Bureau estimates that by 2050 the number of centenarians will be about 834,000.

Now that Americans are living longer, there has been a shift from efforts to medically prolong life to ensuring that a prolonged life is worth living. Some of the most practically applicable work has come from the field of memory and cognition.

“It is cognitive capacity, more than any physical disability that most often determines whether people can attain extreme old age while remaining active,” according to Margery Silver, Ed.D., a neuropsychologist at Harvard.

Through research and clinical studies, psychologists are proving that humans in their later years have far more physical and mental strength than anyone ever imagined.

And more than that, they’re actually designing methods to help people change their behavior to take advantage of increased longevity. In fact, the psychological community is starting to believe that memory and cognitive abilities don’t necessarily decline with age as has been traditionally thought.

A team of Princeton University psychologists found that adults continue to grow new brain cells throughout life. Even these late-generated brain cells, researchers found, may allow older people to bolster their learning and memory capabilities.

Such capabilities were never envisioned under the old theory that brain cells stopped forming and actually started dying by age 40.

“With memory, it does appear that people’s belief systems are important,” said John Cavanaugh, Ph.D., a researcher on aging issues at the University of North Carolina at Wilmington.

Older people, for instance, might believe their memory and intellectual power is insufficient, and therefore may avoid learning how to use a computer or shy away from a training course in strategies for learning and retaining new information.

“The stereotype is that memory is supposed to decline,” adds Cavanaugh, “but that’s still an open question.”

Peter Martin, Ph.D., a professor of human development at Iowa State University puts it even more bluntly, “No matter what your age, the memory is still trainable. You can teach an old dog new tricks.”

Meanwhile, memory loss isn’t the only age-related decline that can be reversed. So, too, can the frailty of old age, said Robert Kahn, Ph.D., of the University of Michigan. Kahn, co-author of “Successful Aging” says that most older people, even the very old and weak, “have the capacity to increase their muscle strength, balance, walking ability and overall aerobic power.”

Many older people tout the value of a daily exercise regimen in maintaining their positive outlook on life and physical health. But, in fact, says Kahn, a major benefit of pursuing a physical exercise program is for its influence on memory.

“Physically active people are most likely to maintain sharp mental ability,” he said.

Memory enhancement also appears to be a potential benefit of a balanced diet, according to new research. While it’s been widely proven that good nutrition enhances overall health, research recently conducted at Tufts University found that men aged 50 and older who had low levels of B vitamins, like foliate and B12, were not as good at performing memory tests as those with higher levels of vitamin B.

Other research in the past several years has also linked mental dexterity to vitamins C and E and to beta-carotene. It is now thought that these antioxidants may prevent damage to the brain’s neurons.

Another key research finding is the need to stay connected with other people. “Psychology can get the word out that certain kinds of behaviors, like diet and exercise, are important,” notes Kahn. “But less appreciated is that people do better if they continue to engage with life and maintain close relationships.”

“There is a definite link between social support and health,” adds psychologist David Myers, Ph.D., author of “Pursuit of Happiness.”

“Those who enjoy close relationships eat better, exercise more and smoke and drink less. Perhaps, a supportive network helps us evaluate and overcome stressful events,” Myers added.

“The mental decline most people experience is not due to the steady death of nerve cells,” Kahn said. Rather, it is the atrophy of connections between nerve cells in the brain. Contributing to such atrophy, he said, are routine behaviors that require little brainpower.

“It’s startling to realize just how predictable and free from surprises our everyday lives really are,” he said.

Thus, he stresses the offbeat and the element of fun. “Do something that challenges and engages your mind,” he said, “not because it’s difficult, but because it’s different from what you normally do.”