It had been just shy of 50 years since Bernie Stamm, 74, was an active competitive athlete, participating in track and field in his high school years. Three years ago, he decided to get back into the game and return to the world of sports, quickly winning gold medals in Senior Olympic events. (Amanda Voisard/For The Washington Post)

To 63-year-old Greg Cooke, seeing Diana Nyad become the first person to complete a 110-mile swim without a shark cage from Cuba to Florida on Monday was nothing short of awesome. Nyad is, after all, just a year older than Cooke, and she reminded him of what is possible.

“I saw that, and I thought, ‘Man, this is a total inspiration,’” he said. “It made me feel like I need to get up and get out there and do stuff.”

Unlike Nyad, Cooke, a government lawyer who lives in Garrett Park, was not much of an athlete for most of his adult life. But when he was 48, he changed his ways. “I had little kids and I was fat,” he said. “I had a 6-year-old, and I didn’t want to be an old slug dad who couldn’t keep up with the kids.” Cooke began running and now completes marathons with finish times in the 31 / 2-hour range.

Most people do less physical activity the older they get. Only one in five American adults overall exercises enough, as defined by guidelines established by the federal Department of Health and Human Services — at least 150 minutes a week of moderate aerobic activity, plus muscle-building activity at least twice a week. By their mid-50s, it is about one in six, and only one in seven after age 65, according to a study by the Centers for Disease Control and Prevention.

But while only a small share of people are exercising enough, the percentage of those exercising after 65 has been rising.

As recently as 1998, only 6 percent of Americans older than 65 routinely exercised up to federal levels, according to the CDC. Over the next decade, it doubled. In the most recent study, done in 2011, 16 percent got enough exercise.

The upswing marks a change in attitude since the middle of the century. In 1954, Jack LaLanne, a fitness and nutrition buff who had an exercise program on television, marked his 40th birthday by swimming the length of the Golden Gate bridge underwater, carrying 140 pounds of equipment.

He did it to prove a point, said his widow, Elaine. “In those days, people thought anyone over 40 was over the hll.”

Recent research suggests that this is far from the case. A 2010 study of muscle tissue from lifelong competitive runners in their 60s showed their leg muscles had almost as many motor units, a measure of strength, as those of active 25-year-olds.

But starting exercise regimens at any age can be beneficial, according to the National Institute on Aging, which runs a Web site to encourage exercise among older people.

Nyad’s accomplishment shows that state of mind can be as important as physical prowess, say experts on the physiology of aging.

“It’s more a testament to her spirit than her body,” said Gayle Doll, director of Kansas State University’s Center on Aging. “She is relentless. Diana Nyad’s message is, you can do incredible things when you’re older. We’ve just been told that we can’t. A lot of people have the body to do it, but they don’t have her indomitable spirit.”

The human body loses protein with age, and with it goes muscle mass. In power sports, such as gymnastics or the 100-meter sprint, people hit their peak in their 20s. But in endurance sports such as swimming or long-distance cycling, athletes don’t peak until their 30s, after many years of training.

“Every time we say there are limits to the human body, someone like Diana Nyad breaks it,” said Frank Wyatt, who teaches exercise physiology at Midwestern State University in Texas. “In most studies on aging, the line goes in one direction, and it’s generally down. Not a lot gets better. But if you look at Diana Nyad, you can say maybe our resolve does.”

Doll said women often are better at endurance sports than men are, because their bodies have more fat that helps fuel their activity.

“You have to work harder, but you can maintain a lot of muscle mass,” said Doll, citing a study in the 1990s in which nursing home residents who did quadriceps training almost tripled their strength.

Carol Mackela, 62, of Arlington was a competitive diver in college, but didn’t dive for 33 years until 2006, when she heard an old teammate from college had taken it up again. “Her dives looked better than in college,” she said.

Looking around in the local area, she at first had a hard time finding a coach who would take her on. One coach “didn’t have time for adults; he wanted to fill his slots with kids who are going to the Olympics.”

But Mackela, a retired government attorney, eventually found a coach. She now competes regularly and will participate Saturday in the Northern Virginia Senior Olympics, along with other divers in their 50s, 60s, 70s, and 80s.

Some things have changed since college. “Most of us are a little heavier, so in somersaulting dives, if you haven’t done it in 30 years, you have to find out where you are,” Mackela said.

Older adults also need to stretch more — and conquer fears that a younger person might not have. “You understand more as an adult what can happen if you do something wrong,” she said.

Bernhard Stamm, 74, of Ashburn learned that lesson three years ago, when he resumed doing field events after a 50-year hiatus and got so enthusiastic that he pulled his hamstring after failing to warm up properly.

“You’ve got to listen to your body,” said Stamm, a retired architect who was a track and field athlete in high school in Switzerland.

With 25 gold medals in senior competitions under his belt, Stamm plans to compete in the Northern Virginia Senior Olympics next week in the standing long jump, running long jump, high jump, javelin, shotput, and softball throw. He’ll even be adding some tricks he didn’t know in high school.

“The Fosbury Flop, where you jump over backwards,” he said, referring to a move popularized in the 1968 Summer Olympics. “That didn’t exist when I was a kid, so two years ago I learned it, and now I’m doing a Fosbury Flop.”

Nyad is a baby boomer — part of the generation born between 1946 and 1964 — and her feat may foreshadow a change in attitudes among a generation that has never liked to think of itself as old.

“She just didn’t give up, she was determined to do it,” Cooke said. “I’m thinking, ‘All right, I can’t let these little aches and pains hold me back; there’s things to do and I’m going to get out there and do them.’ ”