Mid-life: a great place to be.

Because life expectancy has increased so dramatically in this century, the definitions of middle age and old age are undergoing significant change. The postwar baby boomers are entering middle age; the population older than 65 is expected to double by 2007. And thus, because so many of us expect to live a long life, the notion of 65 as the gateway to old age has begun to seem oldfashioned.

Today people may regard themselves as still middle-aged in their late 50s and 60s--the peak of accomplishment and satisfaction for many. People in their 70s and older often retain their health, their energy, and their zest for life. Of course, for older people who are in poor health, lack access to adequate health care, or live in poverty, the picture may not be so rosy.

Nevertheless, recent research has done something to demolish the stereotypes of aging. None of the following propositions would have been taken seriously 30 years ago, yet all are now regarded as valid.

1. Some mental functions may improve as you age. Biologists used to believe that the end of early childhood marked the end of brain development, and that the brain was complete and static until it began to decay. Now we know that the brain is dynamic and adaptable well into middle age and beyond--open to change and to learning, unless disease intervenes.

Anyone over 45 or so is familiar with a slowdown of memory: names and facts may become harder to retrieve, although temporary memory lapses can be ameliorated with training. Yet studies of animals show that the brain remains open to development. Knowledge and good judgment are cumulative; they can and do increase with experience. Studies have found that many mental capacities are surprisingly stable across the years.

One thing that makes a difference is keeping the mind in shape: working, reading, taking classes, talking to others, cultivating a hobby, playing games, traveling. Another thing that can help is regular physical exercise, which seems not only to help preserve neurological functioning into old age, but also potentially to enhance it in older people who have previously been sedentary.

2. The benefits of exercise tend to intensify with age. According to a recent editorial in the Annals of Internal Medicine, "functional aerobic age is probably lowered by the conditioning effect of repeated exercise." In other words, exercise makes you younger. Studies have indicated that exercise prolongs life. In one study of 16,000 men, those who walked nine or more miles a week had a lower mortality rate than those who walked three miles or less. Those who engaged in light sports for two hours a week also increased their life expectancy.

In another study, conducted at Washington University School of Medicine in St. Louis, heart function in men and women in their 60s improved by 25 to 30 percent after a year of endurance exercise. Men in their 70s showed increased muscle strength after just eight weeks of strength training.

A recent report from the Medical College of Pennsylvania cited growing evidence that older people can start an exercise program at any age and have short-term physiological benefits as well as a reduced incidence of chronic diseases.

If you are starting an exercise program in mid-life, you need to start slowly and increase the length and intensity of your workouts gradually. Check with a doctor before you significantly increase your exercise level.

3. Sexuality does not have to wane with age. An active sex life, contrary to myth, is by no means unusual in mid-life or even old age. Ongoing research indicates that 80 percent of men in their late 60s continue to be interested in sex. At 78 or older, one out of every four men is sexually active. Women too retain their sexual abilities and interests into their 70s and beyond. Indeed, sex may be freer and more satisfying, since it is now separated from reproductive worries. Decreased sexual activity in older women usually arises from lack of a partner rather than lack of interest.

It is true that sexual capacity in men can be altered by age, and that women may experience a decline in vaginal lubrication after menopause. But these problems are not necessarily permanent and can be treated medically, if necessary--they need not limit sexuality.

4. Menopause can have a positive side. Menopause, marked by the cessation of menstruation, is a process, not an event. It may bring certain worries to the fore (fear of aging, for instance, or of being defeminized or desexualized). Yet most women find the passage relatively smooth. Some of the changes are for the better: no more menstrual cramps, no need for contraceptives, and no more worry about pregnancy. A woman's interest in sex may actually increase after menopause.

The great majority of women don't have medical problems caused by menopause. But for some women, estrogen loss may result in hot flashes or vaginal dryness--as well as a decline in bone density, putting them at risk for osteoporosis later in life. Most women will want to seek advice from a doctor about hormone replacement therapy, though not everybody needs it.

5. A balanced diet is still a plus. It's just not true that older people need nutritional supplements, or lose their ability to digest high-fiber foods, or should stop eating protein, or have to live on laxatives. Yet these and other myths persist. Older people need the same healthy diet as any healthy adult-plenty of fruits, gains, and vegetables, which supply not only vitamins and minerals, but fiber as well (you need about 25 grams of fiber a day, a level that, when consumed with adequate fluid intake, will ward off constipation for most people and provide other long-term health benefits). Carbohydrates should supply most of your calories, protein should account for 12 percent to 15 percent of your caloric intake, and fat no more than 30 percent. You don't need supplements of zinc or vitamin C. It is true that caloric needs seem to diminish somewhat with the years, beginning as early as age 20 and decreasing by about 2 percent with each passing decade. But this may be partly because people tend to become less active.

6. The empty nest is often a happy nest. The departure of grown-up children from home--once thought to result in severe depression for mothers-may actually improve the lives of parents. One study of women with "empty nests" found that they often emphasize their opportunities to take on new roles, to function assertively and independently, and to enjoy their freedom once the daily responsibilities of parenthood are behind them. Though the study did not look at the effect on fathers, they too might welcome what Dr. John Oldham, a psychoanalyst at Columbia University College of Physicians and Surgeons, calls a "much needed-breather." The nest may quickly be filled up again by dependent parents, or sons or daughters returning with or without young children. Yet the respite can be welcome.

7. Retirement can have positive effects on health. It's a myth that retirement can take years off your life, although many of us worry about being bored or feeling useless when we're no longer part of the workaday world. But studies support the idea that deterioration after retirement, when it occurs, is usually caused by some prior illness or disability. Many people actually thrive on a more leisurely way of life. Retirement, for many, presents an opportunity to reexamine life goals and do new kinds of work.

8. You have the time to be altruistic. Although, according to the stereotype, older people are seen as needy rather than generous, a recent study suggests that concern for others increases with age. An estimated one third of the elderly do volunteer work. In one study, reported in Psychology and Aging, almost 1,400 passersby were observed in a shopping mall as a pregnant woman solicited donations to fight birth defects: older people gave more often than the young, and those 65 and older gave more often than any other group.

As you get older, too, you may have not only the time to be generous but also more available resources. There's some evidence that people who take an active role in their community tend to be healthier than those who live in social isolation. Thus altruism may benefit the giver as well as the receiver.

A recent survey found that many people go through a period of "major reorientation in or by their 40s." More than half of those aged 56 to 65 said they completely agreed that "middle age is a more caring and compassionate time."

9. You finally understand your parents. According to Dr. Oldham, emerging from adolescence into adulthood involves learning to interact with your parents from your own vantage point. Parents, as perceived by middle-aged offspring, are less "magically omnipotent." They become almost peers. As parents grow very old and die, however, children may idealize them once more. "Parents are gradually forgiven for their shortcomings, since one can no longer secretly hope for them to change or make amends for earlier mistakes," writes Dr. Oldham. As people grow older themselves, they tend to see their parents as having done their best, and largely forgive them for their real limitations and mistakes.

10. Life may stabilize in pleasant ways. According to Dr. Elizabeth Auchincloss of Cornell University Medical College, "middle age is not only less eventful but also less stressful than youth .... As men and women are freed from the constraints of social roles that are currently typical of family life, men often shift toward a more nurturing, sensual, and affective orientation, while women, in contrast, begin to explore `more socioexpressive roles.'" In other words, you have a chance to think about and express new aspects of yourself.