President Reagan charged today that Walter F. Mondale "has made a career out of weakening America's armed forces" and has "always found one reason or another for opposing vital weapons systems and the modernization of our forces."

Reagan went on the offensive today as he campaigned across Michigan, but the question of his physical and mental fitness for another term lingered after his faltering performance in Sunday's debate in Louisville.

Reagan's personal physician, Dr. Daniel Ruge, said, "I think he was tired, everybody was tired" during the debate. The White House also released further details of laboratory studies from Reagan's physical examination of last May 18. The statement issued today said Reagan was declared "a mentally alert, robust man who appears younger than his stated age" by Navy Capt. W.W. Karney, an internist who supervised the exam. Story on Page A11.

The president made light of questions about his age: "With regard to the age issue and everything, if I had as much makeup on as he did, I'd have looked younger, too."

Reagan, 73, also said that questions about his age were "desperate reaching" by partisan opponents and that he wasn't tired Sunday night.

Reagan appeared upbeat and cracked jokes about his age today. "I've been 39 years old now for about 31-odd years," he told a high school audience.

White House spokesman Larry Speakes said that Reagan wore no makeup in Sunday's debate, that Mondale did and that Mondale also used a white sheet on the lectern to improve lighting on his face. Reagan's lectern had amplification equipment to improve audibility.

Campaign strategists said Reagan's attack on the Mondale defense record was prompted by new Mondale television ads that show him standing on the deck of an aircraft carrier as F14 fighters take off, discussing "peace through strength," one of Reagan's oft-stated themes.

Reagan campaign spokesman James Lake cited for reporters Mondale votes against a host of defense systems while in the Senate. Lake listed votes against the F14, the CVN-70 class aircraft carrier, the C5A cargo plane, the Harrier jet, the Minuteman 3 missile, the Poseidon submarine missile, the cruise missile, the B1 bomber, the Trident submarine, and the anti-ballistic missile system.

Lake also cited Mondale votes in 1971 for withdrawal of one-half of U.S. forces in NATO, in 1973 for a 40 percent reduction in U.S. troop strength worldwide, and in 1970 against salary increases for the all-volunteer Army.

President Carter, under whom Mondale was vice president, canceled some weapons systems early in his term, including the B1 bomber, but later in his term asked Congress for a major defense spending increase.

Reagan said Mondale had privately opposed the Carter defense spending increase.

"Let me say that from reports of many of the people that were part of that same administration, when President Carter in his last two years felt that he should start redressing the military imbalance, Mondale advised against it," Reagan said.

"Today, we're trying to make up for the shortsighted decisions of the last decade," Reagan told a luncheon at the Ukrainian Cultural Center here, claiming that inflation-adjusted defense spending fell by 20 percent between 1970 and 1980.

Reagan did not mention that that was the period in which U.S. involvement in the Vietnam war ended or that Republicans held the White House for the first six of those years.

"Perhaps the policy makers -- and this twisted logic is still around -- were . . . mistaking weakness for peace," he said, adding that he had "no apologies" for his defense buildup and insisting that Pentagon waste occurred in earlier administrations and he is cleaning it up.

Reagan also seized on Mondale's remark in Sunday's debate that he would "repeal" indexing of tax rates to inflation scheduled to start Jan. 1. Reagan charged that Mondale would raise taxes by $85 billion annually by 1989 and "leave a bottomless hole in the pockets of every working man and woman in the country."

A Mondale campaign spokesman said the Democratic nominee misstated his position on indexing in the debate. His September budget plan called for implementation of indexing for those with incomes under $25,000 a year and "deferral" for others.

"No wonder he goofed," Reagan said later today, "the price of repealing indexing would be enormous.