Credit Ronald Reagan for at least one great achievement. He has made this the Era of the Geezer.

The first president to celebrate his 75th birthday in the White House, Reagan evidently has exercised his executive privilege this spring to repeal the law of diminishing returns for other over-age overachievers. How else can one explain the wondrous events of the past few weeks, including Jack Nicklaus' winning his sixth Masters golf tournament at the age of 46 and jockey Bill Shoemaker riding his fourth Kentucky Derby winner at 54?

Toss in 81-year-old pianist Vladimir Horowitz playing himself onto the cover of Time magazine with triumphal homecoming concerts in Moscow and Leningrad 60 years after he left the Soviet Union.

Add ex-governor William P. Clements (R) of Texas, 69, smashing two young challengers to win another chance at his old job in November. Yesterday, with primaries in Ohio and North Carolina, he was expected to be joined by ex-governors James A. Rhodes (R), 76, and Terry Sanford (D), 68, favored to beat younger rivals for nomination to the governorship and the Senate respectively. The trend is unmistakable.

But that is just the beginning. Don't forget Charlie Whittingham, a respected trainer for 40 years, saddling his first Derby winner, Ferdinand, at age 73. Nor pitcher Phil Niekro, fooling the batters with his knuckleball at 47. Nor husband-and-wife acting stars Hume Cronyn, 74, and Jessica Tandy, 76, lighting up Broadway with their new play, "The Petition."

The yuppies can crow about Dwight Gooden and Roger Clemens and all of their strikeouts. But the truth is plain enough even for those with failing eyesight to see: The standouts of 1986 are notably long in the tooth.

Not even the most diehard of Democrats -- Tip O'Neill, for example -- can deny Reagan credit for inaugurating this current era of geriatric chic. The speaker may be ready at 73 to make way for the next generation of Kennedys to take over his district, but his contemporaries are just catching their second wind in this springtime for the ancients.

It only adds spice to the mystery that the man who every day sets a new record for seniority in the presidency came to us from California, the most youth-conscious, youth-worshiping state in the Union.

Films, television and music videos exalt the energy, the attractiveness and the sheer abandon of the Teens-and-Twenties generation. Hollywood somehow ignores the millions of retirees who have migrated to California's sunny clime. Egg-bald Alan Cranston may be the state's senior senator, but Madonna is its symbol.

Yet out of this curious culture came Ronald Reagan, ex-broadcaster, ex-actor, ex-television host, launching a third or fourth career as a politician more than halfway through his sixth decade, and capturing the White House when he was well past retirement age. At some level of psychic energy, unmeasurable by scientific instruments, his achievement sent a message to others.

Nicklaus won the Masters in 1963, 1965 and 1966, the years when Reagan was campaigning for Barry Goldwater and winning his first term as California governor. He won the Green Jacket twice more in 1972 and 1975, when Richard Nixon and Gerald Ford were in the White House.

Shoemaker's first two Derby wins came -- are you ready? -- during the Eisenhower administration, in 1955 and 1959. Like Nicklaus, he had a win in 1965 as well.

Then Shoemaker waited 21 years and Nicklaus 11, until the Reagan vibes signaled that 1986 was the year for them to cap their glorious careers by becoming the oldest men ever to win the classics in Louisville and Augusta. Both of them had been written off as has-beens. Both of them had been urged to hang it up. Instead, they took on and beat the best of the generation behind their own.

Think that's just coincidence? No way. What Reagan has done for the national psychology is what Roger Bannister did for the world's milers. After he broke the four-minute barrier, dozens of others found they could do the same. He liberated their energies from the shackles of limited expectations.

If you look at the Reagan presidency as an exercise in mass psychology, rather than a game of policy making and implementation, you can see that's what he's been about, right from the start.

That's what the rhetoric about contemporary heroes in the first inaugural was suggesting; it's what he was telling Americans by seating folk-heroes in the House gallery with Nancy Reagan at each of his State of the Union addresses. He was saying: Try something impossible, and you may be surprised what happens.

That's the traditional commencement address message from Old Geezers to young people. But Reagan obviously meant it to apply to superannuated athletes, artists and politicians as well. Except for one thing, I'll bet he is as pleased about the triumphs of the Old Geezers over the supposed barriers of age as anyone in this world.

The only catch is this: Among those who won't yield to the beckoning finger of time are those Old Geezers on the Supreme Court. Don't those guys know when it's quitting time?