The question of whether Ronald Reagan is too old to be president emerged yesterday as an overt issue in his reelection campaign after a Sunday debate performance that even some of his aides described as halting and ineffective.

Democrats, usually sensitive about raising the issue, openly suggested that Reagan, 73, was too old to serve another four years. And a Reagan adviser, commenting on condition that he not be identified, said: "It's the one issue that could change the course of this campaign. We don't think it will, but the potential is now there."

Polls taken for the Reagan campaign showed a 3-point decline in his lead over Democratic nominee Walter F. Mondale since the debate, campaign officials said. The latest Reagan trackings put the president ahead by a 53-to-39 percent margin.

Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee, said Reagan "looked old and acted old" in his nationally televised encounter. When Coelho was asked if the president acted doddering, he replied, "Well, he didn't quite drool."

House Speaker Thomas P. (Tip) O'Neill (D-Mass.), who is 71, said that Reagan "looked tired" in the debate and that age was now a factor in the race.

Top White House officials who work with Reagan daily scoffed at these claims. "It's baloney," White House deputy chief of staff Michael K. Deaver said.

"Some people tried to dredge it up in 1976 and 1980," White House chief of staff James A. Baker III said. "It just won't wash. If you work with him on a daily basis and see how vigorous he is, you know that the statement that he is too old to be president just isn't the case."

Reagan, asked at a Rose Garden ceremony whether age was a legitimate issue, replied, "I'll challenge him to an arm-wrestle any time," in apparent reference to Mondale, 56.

Dr. Richard Greulich, scientific director of the National Institute on Aging, said Reagan is in "extraordinarily good physical shape" for his age.

"In general, his possible shortcomings, if there were any, during the course of the debate were probably as attributable to physical fatigue as to any shortcomings of the mental processes," Greulich said. "But when people get physically tired they become mentally tired as well."

The emergence of the age issue was welcomed in the Mondale camp, accompanied by a recognition that the issue could backfire if pushed openly by the Democrats. Asked whether there was anything Mondale could do to exploit the issue, senior adviser John Reilly replied: "No. We won't go near it -- and we don't have to. It's out there right now and people will decide for themselves how big a factor it is."

Stuart K. Spencer, the president's senior political adviser, gave a similar assessment and added: "The Democrats are treading on dangerous ground with the age issue, because they're making a charge that can't be backed up with the facts."

Reagan advisers agreed, however, that the president's lackluster performance in the closing stages of the debate had openly raised a question that had been only an undercurrent beforehand. They also agreed that Reagan's appearance will be closely scrutinized in campaign speeches and in his second debate with Mondale on Oct. 21.

Throughout Reagan's political career, his opponents have sought to portray him as muddled and uncertain of his facts. In 1980, Reagan's Republican opponents, including George Bush, unsuccessfully tried to raise the age issue.

Until now, Reagan's vigorous appearance has deflected questions about his age. When the subject has come up, it usually has been raised in a humorous way by Reagan, as he did last June at the White House when he observed that President Andrew Jackson "felt pretty trim and fit and vigorous when he left the White House in 1837. I know, he told me."

But this week the age issue began showing up visibly on the campaign trail.

In Cincinnati yesterday morning, Henry Zipf came to a Mondale "citizens forum" with a homemade sign reading, "America Cannot Afford to Have a Senile Pres. Reagan." Zipf, a Mondale backer, said he had watched the debate and was "quite surprised at [Reagan's] stuttering and hesitation."

At the same stop, Mondale supporter Ann Ricks said that a professor of nursing who had watched the debate at the Ricks' home had called the next evening to report what had happened in her nursing class that day.

"She told me the nursing students began discussing senility, and in a few minutes, cited four specific symptoms they thought they had seen in Reagan: incomplete sentences, memory lapse on recent events, reversion to the past, all that sort of thing," Ricks said.

Deaver said statements such as these have been made about Reagan for years and "haven't hurt him before." He predicted that Reagan would make a strong appearance in the second debate and said his ineffective close Sunday was a result of poor preparation and scheduling, "for which I take full responsibility."

The issue of Reagan's age was raised yesterday in the lead story in The Wall Street Journal, prompting questions of spokesman Larry Speakes at a White House briefing. He said the question of Reagan's age "doesn't deserve a response" and was of interest only to Coelho and Rich Jaroslavsky, one of the authors of the Journal story.

But Speakes' view was not shared by his superiors, who responded to the issue throughout the day and tried to play it down.