Visit any gym at midmorning and guess who's pumping iron, sweating through a step class or relaxing with a yoga session? While the rest of the world is hard at work, retirees are hard at play — getting into shape.

That's good news, since only 30 percent of age-related maladies are genetic. The rest can be blamed on environmental and lifestyle factors.

In fact, half of the physical decline commonly associated with age is a matter of disuse rather than overuse.

The moral of this story: Use it or lose it.

Researchers are closely examining the effects of exercise on aging, and all the evidence points to physical activity as a key component to healthy aging. That will only become more important as our population continues to age as a whole. There are more centenarians now than ever before; currently about one-fourth of the population is 55 or older. Regular exercise can not only slow the aging process, but reduce the crushing cost of long-term medical care.

The physical decline associated with age actually begins at around age 30, when physiological function starts declining at a rate of 1 percent a year. But regular physical activity can slow that decline to 0.5 percent.

Clearly, someone who has been active for a lifetime has an advantage over sedentary peers, but it's never too late to start. In one eight-week study, very frail nursing home residents (average age, 90) lifted weights and increased their muscle strength by 160 percent and muscle mass by more than 10 percent. The conclusion: Muscles are just as responsive to strength training in old age as they are in youth.

When experts refer to "older exercisers," they mean an enormous group ranging from 55 to 85 years and up. People in this age group have a huge variety of fitness abilities and goals, says Kathy Wenzlau, fitness director of the Adobe Spa and Fitness Center at Del Webb's Sun City Grand in Surprise, Ariz. Exercise programs are customized to accommodate each participant's needs. Some are new to exercise; others are coming back after a long hiatus.

Either way, these older adults want an active and fun retirement, and they recognize that "exercise is the pathway to that," Wenzlau says.

Exercise programs for older adults vary widely, but every program should incorporate strength training, aerobic conditioning and flexibility.

"My personal philosophy on exercise is to take the individual and work from their limitations, whatever they are," Wenzlau says. "I don't want to take someone who's 85 and put them in a chair and tell them that's the only way they can exercise."

Wenzlau starts by reviewing a detailed medical history on each client. Pre-existing medical conditions and medication can have an impact on an older exerciser's ability to work out. Still, she points out, older exercisers are eager to make progress. They're likely to see benefits quickly, and being physically fit has an immediate impact on their daily activities.

Strength training, whether it's using resistance bands, weight machines or free weights, offers some of the most dramatic results. In fact, strength training is the only activity that can stop or even reverse age-related loss of muscle mass.

Cardiovascular conditioning is another important element. Low-impact activities such as cycling, swimming and walking are among the most popular with older exercisers.

But Wenzlau finds her clients enjoy any new challenge. She's currently working with a group of hikers, age 65 and up, training for a springtime hike to the bottom of the Grand Canyon — and back. Their cross-training program includes hikes up Phoenix's 2,500-foot Squaw Peak, weightlifting and water aerobics.

She's also had a lot of interest in a new in-line skating class (perfect for residents who want to skate with their grandkids), and the community has a number of athletes who train and compete in the Senior Olympics, including a race walker and several cyclists.

Flexibility is also important: Older exercisers should follow a regular stretching routine, but more structured forms, such as Pilates, yoga and tai chi, have huge followings among seniors. All these emphasize balance, grace and good posture.

Since up to 90 percent of joint pain is due to poor posture — just imagine what a lifetime of slouching will do to a body — flexibility training can help correct some of these problems, and offer more physical stability. Stretching should be part of one's daily regimen.

These forms of exercise have another bonus, Wenzlau adds. The smooth, controlled motions of tai chi and yoga also help relieve stress, which certainly doesn't disappear with retirement.