Scientists are reassessing their views on problems that were once considered a routine - and unavoidable - part of aging and reaching new conclusions about the future of the elderly population.

"For one thing, we're learning to separate the effects of normal aging from illness," said Dr. Gene D. Cohen, a psychiatrist and deputy director of the National Institute of Aging in Washington, D.C. "Until the mid-70s we did a poor job of that and problems were dismissed as inevitable. But research on intellectual functions has highlighted major differences."Cohen said senior citizens who remain healthy show very little change or deterioration in intellectual functions, regardless of how old they get. And while there is an apparently normal reduction in reaction time, that is "more than compensated for" by improvements in experience, vocabulary and other areas.

During a series of studies, researchers had subjects age 65 and older play video games. "They learned very well," Cohen said, "and with practice significantly improved their reaction time, which disproves the old-dog-new-tricks theory."

He said study of aging rats gave similar results. Both humans and rats developed new "connectors" in their brains - an organ once thought to be static and capable only of deterioration.

The major conclusion reached by virtually all researchers is "there is no standard pattern of growing old."

Alzheimer's disease, which affects only 6 percent of older adults, is often diagnosed when the real problem is depression or an organic illness.

The cause of Alzheimer's has eluded scientists. In the past 20 years, Cohen said, many causes have been "found," including the recently publicized (and then debunked) theory of too much aluminum in the brain, and genetic and hereditary tendencies. Cohen said a very small number of families do show those tendencies, but even with a "strong genetic pattern, most family members have no history of actually getting Alzheimer's. So the cause it a real whodunit. And the more you know about the disease, the more complicated it gets."

Three of the major symptoms of the disease - lost function, need for great personal care and paranoia - are among the most modifiable. And these same symptoms can result from causes completely unrelated to Alzheimer's.

Studies have suggested that the secret to being healthy into old age, he said, "is staying stimulated. Even old rats responded positively to an enriched environment."

Good health habits and attitude are important going into old age. The personality is very stable, Cohen said, and older citizens have "an enormous capacity for a career and productivity" in later life.

Other medical problems, like osteoporosis, can be almost completely avoided with proper exercise and nutrition. And problems that do occur, if properly diagnosed, can frequently be treated and eliminated.

"Saying that the aging process is nature's way of slowing people down," Cohen said, "is a rationalization. If you're sharp at 83, you're very unlikely to lose that - even if you live to be 100.

"Growing old doesn't have to mean growing slow or growing ill," he said. "As we learn more, I think we'll find the future of the elderly is very different than most people imagine it."