As Democratic politicians and news-media commentators focus for the first time in the presidential campaign on the question of Ronald Reagan's age, political experts say they expect this to be a factor in the election only if the President does poorly in the second debate with Democratic candidate Walter Mondale.
''You will not hear Democrats saying that Reagan is too old to do the job, but it will be there at the whispering level,'' says Austin Ranney of the American Enterprise Institute.
The President himself has gone on the offensive. Responding to a question as he departed for a campaign swing Wednesday, Reagan said he ''wasn't tired'' in the debate and ''if I had had as much makeup'' as Mondale in that television encounter, ''I'd look younger, too.''
Meanwhile, the White House physician, Dr. Daniel Ruge, pronounced the President's health to be ''excellent.''
Experts agree that it is not chronological age that is at issue but whether the President is evidencing signs of decline.
It is not unusual for the President to hesitate or falter in an unscripted event, such as a news conference. The debate was the first time he performed at such length without a prepared text.
But age is a potential concern in the minds of voters, says Professor Ranney. ''If Reagan performs as badly in the second debate as he did in the first, the speculation will be redoubled and it would become a factor if not an issue in the election. If he handles himself well, that would be a good development that would quiet things down.''
''Suddenly the expectation level for Reagan has changed,'' comments Curtis Gans of the Committee for the Study of the American Electorate. ''Now he has to hold up in the debate. The perception in the first debate was of a person who was old and not in control of his memory. If that is reinforced by the second debate, age could become an issue.''
Since the Mondale-Reagan encounter in Louisville, the subject of age has received conspicuous attention in the news media. It has been discussed on television news programs and by commentators. The Wall Street Journal this week ran a front-page story on whether the President was showing his age. All these reports flowed from Reagan's performance on Sunday.
Some observers, however, believe the issue is being overstated. The significance of the debate lies not in how poorly the President did, they say, but in the fact that Mondale unexpectedly did so well. Moreover, polls do not yet show that even if Reagan supporters thought Mondale won the debate, they would change their vote in November.
''Reagan's leadership image is very strong, and it will take a lot to overcome that,'' says Andrew Kohut, president of the Gallup organization. ''The Democrats may latch on to this, but I can't see it as being an effective approach, because our polls traditionally have shown that people have not seen Reagan as hard-working but as someone with his own style in getting things done and affecting the course of national events.''
John Sears, a former Reagan political adviser, suggests that the debate may have jarred some people, but not enough to work against Reagan at the polls. ''I've never seen a situation where age is enough to do a man in,'' he says. ''People are interested in competence. The Eisenhower situation in the 1956 election was worse. He had had a heart attack and his speech was flawed. Obviously it did not hurt him a bit.''
Officially, the Mondale campaign is staying clear of the age issue. Mondale will not capitalize on it, spokesmen say. But Democrats, including House Speaker Thomas P. O'Neill Jr. and Rep. Tony Coelho (D) of California, have remarked that Reagan looked tired in the debate and that age was now a campaign factor.
Ann Lewis, political director of the Democratic National Committee, states that every report coming in from the field since the debate indicates that the public is becoming concerned about Reagan's abilities and competence. ''It's out there and it's starting to come out in the polls,'' she says. ''It's an increasing topic of concern - even older people are raising it and being more open.''
Even while they may be quietly pleased by the emergence of an issue that could work on Mondale's behalf, Democratic leaders stress that the issue could backfire on their campaign. They feel that their party has been in the forefront of dropping age barriers to employment and fighting discrimination. ''So this could cut against the Democrats as well,'' says a Mondale campaign official. ''There are a lot of senior citizens who are very sensitive about being asked to step aside from their jobs because of age.''
White House advisers and Reagan campaign officials say the Democrats should not make age an issue, because it isn't true. They maintain that the President is vigorous and strong, and will turn in a good performance in the upcoming Kansas City debate.
But there is an underlying unease at the White House and among many Republicans.