When an average man reaches his middle 30s, his body starts to give signs that the last of his best years are giving way to the first of the worst.

For a ballplayer, age 35 is already a time of suppressed terror.

He knows, because a hundred years of history tells him so, that one of two things will happen. He'll lose his skills suddenly and never begin to get them back, or, if he's lucky, he'll find some way--desperate, redoubled hard work or technical adjustments--to add a few years to his career as a sort of 90 percent imitation of his former self.

Ken Singleton and Al Bumbry of the Baltimore Orioles, both 35, have hit this frightening wall of age and they've hit it hard. In these soul-testing days, they are playing with perplexity in every step, struggling and stumbling through a pennant race with self-doubt swirling outside them and within them.

Manager Earl Weaver has been tormented for months trying to decide when, or if, they should be benched or platooned. Couldn't Gary Roenicke or rookie John Shelby do a better job in center? Shouldn't Benny Ayala be the designated hitter against right-handers?

In the end, Weaver has let both veterans play as much as ever, crossing his fingers and looking for signs of a breakout which hasn't come.

It is just such cruel situations, watching veterans go bitterly over the hill, that, as much as anything, have driven Weaver to retirement.

Weaver hates these annually repeated scenes: years of good times and mutual respect going down in flames of acrimony. Only one Oriole blamed his muscles, not his manager, when he took the long slide -- Brooks Robinson.

"I'm tired of steppin' on players' toes and getting myself all upset," Weaver said Friday. "I don't want to do it no more . . I'm to the point where inside of me, I have doubts that I still can . . . I can find somethin' more enjoyable than hurtin' people's feelings."

Teammates sympathize, almost agonize, with both their September sufferers; Singleton and Bumbry epitomize the Oriole ethic of hard work, gentlemanly behavior, applied intelligence and years of proven clutch ability.

Yet, every time Singleton deflates a rally by grounding into a double play left-handed or striking out pathetically right-handed; every time Bumbry plays a fly ball into a hit, or lets a mediocre runner go first to third on his arm, or gets overpowered by a left-handed pitcher, or can't steal a base when it's time to steal, a shudder goes though the Orioles.

They're pros and they know the hard, unfair fact: Kenny and the Bee, their favorite people, are killing the club, have been killing it all season.

Just two years ago, Singleton and Bumbry were at their peaks. They were at the heart of Baltimore's back-to-back 100-win seasons.

In those two seasons combined, Bumbry averaged .302 with 100 runs scored, 40 steals and 250 times on base per season; not too many played center better. Now, in 535 at bats, Bumbry is hitting .255 with 72 runs, eight steals and a paltry 178 times on base.

In 1979 and 1980, Singleton averaged .300 with 30 homers, 108 RBI and 101 walks. Now, in 533 at bats, he's hitting .251 with 13 homers, 73 RBI and 82 walks; his slugging percentage is down more than 110 points and his on-base percentage more than 60. In his last 200 at bats right-handed, Singleton's average is under .185 with no homers.

What makes the situation so painful and so typical is that Singleton and Bumbry are convinced that only small, correctable injuries stand between them and their former selves.

For two years, Singleton has cited various muscle problems in his right elbow, forearm and, now, palm. He's tried various rehabilitation programs. The most he'll admit is, "If I had it to do over, I'd never have had that (ulnar nerve transplant) elbow operation in '77." Problems ever since.

Veteran observers suspect Singleton has the same problem that afflicted Babe Ruth, Lou Gehrig, Jimmie Foxx and countless others; one day, they can't catch up to a major league fast ball.

Singleton disagrees. "I've sure got the league set up. Next year, everybody's going to throw me fast balls, and, when this hand gets back to normal, I'm going to be waiting for them."

Bumbry has been tormented by leg injuries which have curtailed his stealing, rob him of infield hits and prevent him from compensating for outfield misjudgments with speed. Bumbry says you can't get healthy when you have to play every day and, next year, his legs will rebound.

Even Bumbry's luck has been lousy. Around midseason, Bumbry got a box of light 31-ounce bats and changed the book on himself, pulling the ball with authority, even slamming three leadoff homers in the first inning in a week.

But, soon, the bats broke. The replacement shipment was just more of his old 33-ouncers. For weeks, Bumbry hasn't been able to get the bats he wants. Maybe, for Reggie Jackson, special arrangements. For Bumbry, watch the mail.

"You wouldn't think two ounces would make so much difference, but it does," says Bumbry. "I can get around on the ball with one, but not the other."

The Orioles have decent potential replacements for both their former all-stars. Still, the Birds ask themselves, who do they have who'd be as good as Bumbry and Singleton if they came back close to form? The answer: nobody.

Last season Jim Palmer was 35 and, with a 7-8 record, didn't even qualify as offseason trade bait.

Bumbry and Singleton are hard workers with "young" bodies. They're smart and may adjust, as Palmer, 14-4, has.

They also represent a perennial problem that has no solution, only pain and the certainty that time will give its answer.