Pick 100 adults. If they are a true snapshot of the American public, 25 of them share Mark Twain's view of physical activity ("Exercise is loathsome"). At least 35 are semiactive–they may amble around the neighborhood or do light gardening but rarely get their hearts pumping at a good clip. That leaves no more than 40 people who can be called fit.

Abe Weitzberg and his father, Oscar, fall into an even smaller cluster: people who have been fit for a lifetime. The younger Weitzberg plays soccer twice a week, often with teenagers. His father does flexibility and strength exercises every day for half an hour. A handball fanatic, he took up tennis in the late 1980s and played until recently. He had no more regular partners, and no wonder: He is 93. His son is 65.

Not everyone can keep up with this pair from the Washington, D.C., suburbs–but just about any healthy person can become and stay fit. Those who do will probably feel better, live longer, find it easier to look in the mirror, and defy the stereotype of a feeble, bedridden oldster. The American College of Sports Medicine (ACSM) says a lifetime regimen should combine three types of exercise:

Aerobics. Running on a treadmill, playing soccer, or other endurance activities that push the heart to at least 55 percent of its maximum rate (story, Page 66) for 20 minutes or more at least three days a week.

Strength. Doing eight to 10 weight or resistance exercises to condition the large muscles, eight to 12 repetitions of each, at least twice weekly.

Flexibility. Stretching the major muscle groups at least two days a week.
The mix and emphasis of activities should change over time. An older person's program, for instance, needs to tilt toward strength and balance to encourage mobility and avoid falls. The age ranges that follow suggest areas to stress–and common mistakes that can derail a fitness program.
 

20 TO 40
Young and restless
Special focus: building a foundation in aerobics, strength, and flexibility
Beware of: too much, too soon

This group, says fitness consultant Ken Baldwin, a former member of the Governor's Committee on Physical Fitness and Sports in Massachusetts, "sees muscle magazines, gets impatient for this look . . . and does things like throwing around too much weight." A study recently published in Physician and Sportsmedicine showed that between 1978 and 1998, the rate of injuries related to weight training in men ages 25 to 44 rose by 86 percent.

You can lower your risk of injury by hiring a personal trainer, at a cost of $25 to $100 an hour. A good trainer will start by asking about physical or medical problems that might interfere with training or could worsen if stressed. Look for a trainer certified by the ACSM or the American Council on Exercise (www.acefitness.org).

After five or 10 sessions over the first two or three weeks, you can scale back. Ray Kieffer Jr., 36, an orthodontist in San Diego, had a personal trainer instruct him in the intricacies of weight training; he now sees her every two weeks to fine-tune his program. He goes to the gym twice a week for 60 to 90 minutes of weight training. For endurance, he runs 3 to 5 miles once or twice a week and swims laps in a pool once every week or two. He does yoga for flexibility, which is important because of his work. "My neck position gives me a problem because I'm always leaning over," he says. Regular deep massages also help work out the kinks.
 

40 TO 65
Weight creep
Special focus: replacing lost muscle
Beware of: short-term dieting

Most people recognize the unhappy tendency to gain about 10 pounds a decade after age 40. But they don't realize that they are losing muscle mass at the same time, says Wayne Westcott, fitness research director of the South Shore YMCA in Quincy, Mass. The 10 pounds, he says, is the net gain from adding 15 pounds of fat and losing five pounds of muscle.

As weight creeps up, the natural reaction is to try to diet it off, and a nasty, self-defeating cycle kicks in. About a fourth of the weight loss is muscle. Each lost pound of muscle depresses the body's metabolism by about 40 calories a day. How unfair: Drop 20 pounds, including 5 pounds of muscle, and the lost weight will return unless you cut 200 calories a day from your food intake. Trying to burn off the excess weight with aerobic exercise rarely works. You'll get frustrated, pile on still more hours of running or stair climbing, and succumb to injury or exhaustion.

Strength training can replace the lost muscle, get the body's metabolism back up, and keep weight off permanently, says Westcott. A few years ago he enlisted 1,132 people ages 21 to 80 in a program consisting of two or three sessions a week of 25 minutes of Nautilus strength training, 25 minutes of aerobic exercise, and nutrition education. After eight weeks, all of the participants had lost fat and gained muscle–4.4 pounds of fat loss and 2.3 pounds of muscle gain, on average, for those from age 41 to 60. Two sessions a week worked nearly as well as three, and Westcott later found that even one session offers 73 percent of the benefits of three times a week. "You certainly can find one day a week to do Nautilus or some other strength training," he says.

A regimen that included strength training has worked for Mike Moss, a 56-year-old Wheaton, Md., administrator who didn't like what he saw in photos from his oldest son's wedding three years ago. "I was embarrassed," he says. He now works out three times a week for 30 to 45 minutes on various weight machines, followed by an hour on an elliptical cross-trainer for endurance. His weight has dropped from 190 pounds to 155 to 160. As a bonus, he is better able to hold, aim, and reload the heavy black-powder Civil War-era weapons he shoots competitively.

But the ongoing boom in strength training has driven up emergency room visits in this age group, too. Most mishaps occur at home. If you can't afford a trainer, use a guide, like Strength Training Past 50 by Westcott and Tom Baechle ($16.95).
 

65 TO 80
Keeping active
Special focus: maintaining flexibility and strength
Beware of: fear

Because of lack of exercise and age-related muscle loss, "many older people are extremely unfit and very weak," says William Evans, director of the geriatrics department's nutrition, metabolism, and exercise program at the University of Arkansas for Medical Sciences in Little Rock. Making them stronger, he says, reduces bone loss and improves their ability to live an active life.

Group sessions can help motivate older people who have been sedentary. Since 1994, Pennsylvania's Department of Aging has run PrimeTime, a program of exercise classes for older individuals. Mary Frances Sears, 67, has been faithfully attending classes at the Philadelphia Senior Center for three years. In 45-to-60-minute sessions three times a week, she uses elastic bands and small dumbbells to increase upper-body strength, and stretching and bending routines to enhance flexibility. She also goes to a track once a week to walk–a real triumph, because her arthritic knee was replaced last year. "I feel more energetic than I did even when I was working," she says. Sears and a friend who works out with her have vowed never to cancel except in an emergency.

No programs or classes nearby? Get Exercise: A Guide from the National Institute on Aging. It is a free self-help book that explains recommended exercises in words and clear diagrams, offers sample workout schedules, and includes self-assessment tests (800-222-2225 to order). The book plus a companion exercise video cost $7 postpaid (send check or money order to NIAIC, Dept. US, PO Box 8057, Gaithersburg, MD 20898-8057).

A study in the current Journal of Gerontology, based on a survey of 327 women at least 70 years old in the Vancouver, Canada, area, found that many elderly women believe exercise is too risky. Reactions to a slow stretch reaching for the toes included "might hurt spine" and "pull a vertebra." Such fear, of course, is far more paralyzing than stretching could ever be.