PHILADELPHIA >> Seventy-seven-year-old David Pallett, who began exercising seriously about four months ago, started a recent workout by donning a 15-pound vest and climbing 100 stairs — two at a time.

After that little warmup, personal trainer Jim Hart, who specializes in working with older adults, led Pallett through an hour of exercises meant to improve strength, balance, power and metabolic health. The semiretired lawyer gamely worked his abs while perched precariously on a 72-centimeter ball. Hart combined such movements as punches and lunges so that Pallett was using his arms, abs and legs all at once. That required the kind of whole-body coordination needed to avoid falls or do physically demanding work at home.

Pallett, a trim man with a white beard and silver hair, has increased the weights he’s using by about 30% since he began these workouts. Hart thinks his client is still in the “beginning stages of his potential.” It will likely be at least six more months before Pallett plateaus.

Could he catch up to similar men who have exercised their entire lives?

Most experts say people who put off exercising until their retirement years are at a disadvantage. They enter late life — a time when strong muscles and good aerobic capacity can make the difference between independence and disability — with poorer-quality blood vessels, nerves and muscles than peers who have always been fit. New exercisers can repair much of the damage, but probably not all of it.

The good news is that you don’t have to catch up to the lifelong runners and gym rats to improve your health and quality of life. “You can take really unfit people at 70 and get them really fit and doing amazing things,” said Dan Ritchie, president of the Functional Aging Institute, where Hart trained.

Pallett would like to live longer than his mother, who made it to 97, and avoid the dementia that took his father in his early 80s. For now, he’s happy that his posture is improving and that his shirts fit tighter across the chest as he’s gained muscle.

Physical activity is one of the most important things people can do to boost the number of healthy years in their lifespan, and experts say it’s better to start young.

“I’m a huge fan of exercise, because, without question, it’s the most effective means that we have today to counter the fundamental biology of aging,” said Nathan LeBrasseur, a physiologist and physical therapist at the Mayo Clinic in Rochester, Minn., who studies muscle growth and metabolism.

Aging drives dysfunction and disease. Exercise can slow it down. Obesity, which often accompanies low activity, accelerates it.

People reach their physical peak about age 30, said Steven Austad, senior scientific director of the American Federation for Aging Research. We lose about 30% of our muscle mass and 50% of strength in later life. Exercisers sustain higher levels of mass longer, and mass correlates with strength, so they start their decline from a higher point than sedentary peers.

“You want to walk into your 80s with as much muscle mass as possible,” said Kevin Murach, an exercise physiologist and muscle biologist at the University of Arkansas. His recent research — in mice — suggests that people who exercise in early life but take a long break might build muscle more quickly if they start again than those who have never exercised.

Increasing numbers of older Americans have exercised for decades thanks to fitness trends when they were younger, researchers said. That has given physiologists a group of high achievers to compare with lifelong couch potatoes. The exercisers are clearly stronger and healthier. Scott Trappe, director of the Human Performance Laboratory at Ball State University, said that longtime exercisers have a bigger physiologic reserve that helps them bounce back from illness or injuries in their retirement years. Lifelong exercisers in their 70s have cardiovascular capacities that are physiologically similar to those of active people 30 years younger, and their muscles have enzymes involved with aerobic metabolism that are the same as exercisers in their 20s, he said.

It is true that some fitness fanatics enter their retirement years with bad knees or pain from injuries. Sedentary people who are not obese could have healthier joints and fewer injuries, but their joints could suffer from the lack of strong supportive muscles. Sedentary people have more back trouble and more cardiovascular problems that can limit capacity.

The composition of muscle changes over time, with fewer and shorter fibers. In heavier, more sedentary people, fat deposits can make muscle look like marbled steak, and Trappe said muscle cells become less able to process energy. Add this to the fact that the ability to use protein from food for muscle building is blunted in aging bodies, said Paul Coen, an exercise physiologist for AdventHealth in Orlando.

Lebrasseur said people who study muscles have long been consumed with age-related decline in muscle mass, but are starting to look at other factors. “Have we oversold the importance of building mass as opposed to building muscle quality?” he wonders.

By that, he means that muscles don’t operate independently. They need a good blood supply and well-connected nerves that tell them when to contract and relax. These things decline with age, too.

A healthy brain is key to strong muscles because that’s where the signals that control muscles start, said Brian Clark, an exercise physiologist who directs the Ohio Musculoskeletal and Neurological Institute. “The muscles are the puppets of the nervous system,” he said. Our brains typically atrophy with age, and that can make habitual motions like walking more challenging, a reason that older people find it harder to walk while checking their phones than their grandchildren do.

The best activity for your brain and heart is aerobic exercise.

The heart supplies your muscles with nutrients and oxygen. Exercisers have more muscle capillaries and more supple arteries than the sedentary. Neel Chokshi, a cardiologist and medical director of Penn Medicine’s sports cardiology and fitness program, said the amount of oxygen your body can use during exercise is a sign of cardiovascular efficiency, and that declines over time.

At any age, exercise as simple as walking can help people avoid catastrophic falls and stave off the day when they’ll need a walker or wheelchair. “There’s never a time in your life when increasing your physical activity is not beneficial,” Austad said. It also can be pretty gratifying. “The thing about weak flabby muscles,” he said, “is if you change them just a little, it can have enormous impact.”

Experts pointed out that you’ll do better if you combine exercise with nutritious food and adequate sleep.

Fran Schwartz, 82, started exercising for the first time about a year ago and now does multiple classes and personal training. She’s pleased that she’s gone from using 2- to 3-pound weights in the strength-training class and from 8 to 14 pounds for chest presses. Schwartz, who has had two hip replacements, says her flexibility, balance and strength have improved. She wants to keep that trend going. “I haven’t even thought about long-term goals,” she said. “I’m just taking it gradually and feeling better.”