ONCE UPON a time it was thought that there was an age -- somewhere in the vicinity of 65 -- beyond which political ambitions were unn that were considered potentially infirm or out of touch or just plain over the hill. Ronald Reagan has, it seems, changed all that. "I will not make age an issue in this campaign," he said in his debate with Walter Mondale in Kansas City. "I am not going to exploit for political purposes my opponent's youth and inexperience." No president before Mr. Reagan served in office past age 70. He was reelected to his second term at 73.

Now a couple of the Republican Party's most experienced war horses are trying to do something similar in two of the nation's largest states. In Texas, former goernor William Clements, at age 68, has announced his intention to run for the office he won in an upset in 1978 and lost in an upset in 1982. Mr. Clements is a man of considerable accomplishment, both in business and government (he was deputy secretary of defense in the Ford administration); he is also a man of sharp temper who does not suffer those he considers fools gladly -- a habit that is sometimes unhelpful in a business in which you often find yourself needing support from just such people. Mr. Clements, who has shown the willingness to spend $8 million or so of his own money on a campaign, is entering this race even though two serious Republicans, Rep. Tom Loeffler and former Boll Weevil Democrat Kent Hance, were already poised to run.

If he does run and win at 69, he will have done no more than James Rhodes did in Ohio eight years ago; and this year Mr. Rhodes, at 76, appears to be running again. Sometimes it seems that only Ohio's constitutional prohibition against a third consecutive term has prevented Mr. Rhodes from being governor more or less forever. He was elected in 1962 and 1966, ran unsuccessfully for the Senate in 1970, was returned to the governorship in an upset at age 65 in 1974 and was reelected in 1978. That year he beat the current incumbent, 47-year- old Richard Celeste. Mr. Celeste takes a Rhodes candidacy very seriously.

He should. Mr. Rhodes is one of those old-fashioned war horses who is blessed with an instinctive understanding of politics and a feel for his constituency. One of the problems the Republican Party faces is that it seems to have fewer such instinctive politicians than the Democrats; many of its candidates, such as Mr. Clements, are people of great accomplishment and talent, but who sometimes lack the tact and deftness that are useful in public life. Mr. Rhodes, like Mr. Clements, will probably have primary opposition, and neither man is a sure thing for the general election. But it's comforting that in a nation where we are assured, whether the subject is politicians or soft drinks, that the public is athirst for anything new, there's still a supply and quite possibly a demand for some talented old political war horses.