The hitters approach Tim O’Brien sheepishly after he has made them look foolish — bewildered them with off-speed stuff or slipped a deceptively fast third strike past them on one corner or the other.

Thirty-five years his junior, young and proud of their hitting skills, they want to know about the white-haired gent on the mound who has shown them up again.

“I’ve had so many guys come up to me after the game,” O’Brien says with a chuckle. “ ‘Sir, I mean no disrespect, but how old are you?’ ”

“ ‘This year I’m 63.’ ”

“ ‘Damn.’ ”

“Just ‘damn.’ ”

At an age more common to managers and coaches, O’Brien is still pitching regularly in the DC Metro Senior Baseball League, a feat noteworthy in itself. But consider that he plays on four teams — in the over-25, over-35, over-45 and over-55 divisions — shutting down hitters not much older than his grandchildren, and you can begin to understand his accomplishments.

“When we play against younger kids, their eyes light up,” says Jerry Klemm, the league’s commissioner and a former teammate of O’Brien’s. “They see a 63-year-old fellow coming out to the mound, and they’re ready to hit the ball 600 feet. So [O’Brien] throws it a little faster and a little slower, moves it around a bit, and before you know it, the game is over.”

O’Brien says his fastball recently was clocked at 70 miles per hour — it’s actually a little faster than it was a few years ago, he says — and he is one of those rubber-armed guys who can pitch as many innings as his teams need.

Although his age and the velocity of his pitches are not that uncommon for senior league ballplayers, says Steve Sigler, founder and president of the national Men’s Senior Baseball League, his ability and desire to compete with younger guys is.

The game “really perpetuates your youth,” says Sigler, who leads an organization with 45,000 players in 225 leagues across the United States, including some over-75 teams. The Washington area league is the second-largest in the country, with 80 teams. Players “think in perpetuity. They don’t think, ‘I only have this many years left.’ ”

This being a fitness column, I was most interested in O’Brien’s longevity and was surprised to find that, beyond a few routines he follows assiduously, he isn’t the kind of workout freak I expected him to be.

He works diligently with weights and resistance bands, and does a lot of stretching. He does no cardiovascular work. He didn’t start pitching until about 20 or 25 years ago (he was a catcher through college).

“Don’t stop moving,” he tells me over dinner one evening. “As soon as you stop, it’s hard to get moving again.” O’Brien walks this talk, taking just four weeks off from his various baseball activities each year. His career as an accountant and a consultant leaves him plenty of time to indulge his passion.

In January and February, O’Brien, who lives in Fairfax, works indoors at a club with W.T. Woodson High School ballplayers, whom he helps coach during their regular season, which begins in late February. From March to May he works with them five or six days a week. His own playing season runs through the summer — he’s in the playoffs right now — and it’s not uncommon for him to play five or six games a week.

This year, for example, O’Brien has thrown 202 innings, 63 of them in the over-25 division, with 128 strikeouts, 31 walks and a 3.47 ERA, according to statistics he keeps. (The numbers include O’Brien’s play for a fifth team in a separate league). In the fall, O’Brien often plays with All-Star teams that compete in MSBL World Series tournaments. He has been part of two national championship teams. His only break from baseball comes in November and December.

“I’m convinced that a lot of people can do this,” he says of continuing to master a sport as we age. “But they just don’t think they can.”

O’Brien credits genetics, what he calls “a strong life force,” for his good fortune, but Glenn Fleisig, who has made a career studying pitching injuries as the research director of the American Sports Medicine Institute in Birmingham, Ala., says several factors are critical in preserving a pitching arm.

Heredity is certainly important, as are good mechanics and the number of pitches thrown at maximum effort. But by far the most crucial issue is whether a pitcher heeds the signals from his body that he has reached his fatigue level. Those who ignore that feedback and keep on throwing are 36 times more likely to be injured than pitchers who don’t, Fleisig says.

“The number one important thing is fatigue, the feedback from the brain, whether it’s Little League or senior baseball,” he says.

O’Brien knows his arm will give out one day, that he will have to move to a position in the field or find another way to remain a Boy of Summer. Usually the thought flits through his mind as a new season begins.

“The first time or two on the mound, I always wonder, ‘Is this the year I can’t do it anymore?’ ” he says.

“It hasn’t happened yet.”