Nancy Reagan said today she was personally hurt by the questions raised about President Reagan's age and competence but is confident he stilled any doubts on those subjects during Sunday's debate in Kansas City.

As Air Force One left Kansas City this morning, the first lady made an extremely rare visit to the rear cabin to talk with reporters about the debate, the campaign and the recent visit of Soviet Foreign Minister Andrei Gromyko.

Dozens of Reagan subordinates had been dispatched to give the president's performance a positive "spin" after the debate, but Mrs. Reagan offered the most telling clue of renewed confidence in the Reagan camp.

She rolled an orange down the aisle of the plane as it took off.

It was the ritual with which she began hundreds of campaign trips in 1980 -- but one she has neglected all this year.

Mrs. Reagan has long been viewed by presidential advisers as a window on her husband's emotions, and she confessed today that Reagan seemed "off stride" in the Louisville debate Oct. 7. But she denied she had asked presidential aides at the time, "What have you done to Ronnie?"

The questions raised about whether Reagan was fit to serve another term are "not true and he showed it's not true . . . I think it's insulting, too, to an awful lot of people," referring to the elderly.

"It's simply not true. After a while, you get used to hearing that from the other side. The American people obviously don't think it's true. You never get used to it."

The 90-minute debate Sunday night -- in which Reagan quipped that he wouldn't raise the age issue to exploit the "youth and inexperience" of Mondale -- "put it to bed once and for all," she said.

White House chief of staff James A. Baker III said Reagan did not use the line in debate rehearsal, but Baker said he had heard Reagan use it before. Mrs. Reagan said she hadn't heard it before.

The line was the turning point in the debate, she said. "I detected a change in Mr. Mondale."

Nancy Reagan was careful not to criticize Mondale directly, however.

Asked how Mondale looked in the debate, Mrs. Reagan said to a reporter, "I couldn't quite hear you." The question was loudly repeated. "I still don't hear you," she said, smiling. Asked if Mondale should "fire his makeup man," she responded, "That's between Mondale and his makeup man."

Mrs. Reagan said the president would like to go to Moscow during a second term. "I'd love to," she said. "He's talked, you know, generally, after the meeting with Gromyko . . . if they have a leader who stays around for a while."

Then, announcing a news "flash" with mock surprise, she described "the whole story" of Gromyko's toast to her during his visit.

"The whole story was that Gromyko came in with my husband for lunch and he toasted me. He had cranberry juice and I had soda water. Big drinkers. Then we went into small talk about the metric system. And then he turned to me and said, 'Is you husband for peace or war?' And I said, 'Peace.' He was a little bit surprised . . . and he said, 'You sure?' And I said, 'Yes.' He said, 'Then why doesn't he accept our proposals?' And I said, 'Which proposals?' and with that people came up and it was interrupted. And then it was time to go to lunch and he turned to me and he said, 'You whisper peace into his ear every night.' And I said, 'Oh, I will. And I'll also whisper in your ear.' "

Gromyko "smiled," she said.

Questioned whether she had decided what Reagan's arms control policy should be in a second term, Mrs. Reagan said, "I'm asking Amy."