"Going on 52," maybe, but 75? Not the way Nancy Reagan sees Ronald Reagan on his diamond jubilee birthday.

"I go along with what he's said, that they mixed the babies up in the hospital, that he's not this old at all . . . " It's a typical Reagan story, one he's told many times, and Nancy Reagan, who usually can't remember a joke, delights in retelling it.

"And I agree with him," she says, her voice bubbling with laughter. "I think they must have!"

She's seated in the White House Map Room, having altered her schedule to talk briefly about her favorite subject: Ronald Reagan, who upstaged all his predecessors when he hit 70 years, 99 days (surpassing Ike, who was 70 plus 98 days when he left office). His age is of endless fascination to people, and Nancy Reagan, 64, knows it. She's gotten over being defensive about it.

"We were looking at a picture the other day in an old magazine of the two us at the old ranch," she says. "I swear he doesn't look any different . . . "

But if they didn't mix up the babies, then he is 75 . . .

"Going on 52," she interjects.

Right. So how does he do it?

"I guess it's not thinking about it. I suppose (that) is a large part of it. He never does. I never do." Their 34th wedding anniversary is a month away and she says she never thinks of it in terms of years. "I never think '34 years! Gosh, where have they gone?' "

Reagan, of course, does think about age if he sees a chance to poke a little fun at himself. A Harvard anthropologist told him at a White House dinner in 1984 that the Chinese had found a 3,000-year-old body packed in a pink jelly. When the jelly was removed, the body's flesh actually was springy. Reagan's mouth dropped open in wonderment.

"What's in that jelly?" Reagan asked. "I'd like to sleep in it!"

Nancy Reagan says there are other things Ronald Reagan never thinks about. One is clothes. "He's very casual about clothes, always has been. He'll say 'I'm going to wear my new suit' and the 'new suit' turns out to be 15 years old." He gets a new new suit when she decides something has to be done. He chooses the suit, also the color. "He does not like gray, doesn't think it looks well on him."

This morning, for instance, if Ronald Reagan was true to habit, he didn't spend one extra minute deliberating about what to wear. "He never has," Mrs. Reagan says.

He awoke to find on his bedside table the first of several birthday cards Mrs. Reagan scattered around the house for him (by last night he had already received 885 greetings from individuals and 800 from school classes, including a 4-by-6-foot card from the Ronald Reagan Scholars at his alma mater, Eureka College). No gift, though. That comes tonight.

Instead of the routine breakfast-a-deux with the morning newspapers in the family quarters -- a meal of juice, fruit, bran cereal and decaffeinated coffee, varied once a week with the addition of eggs -- they were booked into the annual Congressional Prayer Breakfast at the Washington Hilton. On a typical day breakfast would be the last she sees of him until evening unless there's a special event, because he never comes home for lunch -- or for rest.

"All this talk about napping . . . I guess that's gone by the boards. I haven't read that lately," she says.

"Don't bring it up," advises Elaine Crispen, her press secretary who is sitting across the room.

They rarely talk during the day, she says, because she doesn't call him and he doesn't call her. "Unless there's something terrible, like (the space shuttle Challenger explosion) the other day. He didn't know I had seen it live. He said 'Do you know what happened?' and I broke in and said, 'Yes, I was watching.' "

After work, "he just comes home." Around a quarter to 6, right into the bedroom, where she usually is at the time. That's when he does his exercises, the same ones he's been doing for the past five years, since the doctor told him after the assassination attempt that he had to build up his chest muscles.

After that, they watch the network news shows, then have dinner, sometimes on trays, sometimes in the family dining room. There's no cocktail hour. Lately, they've seen more movies than usual (Sunday night it was Woody Allen's "Hannah and Her Sisters"), but at Camp David, not the White House. During the week, she says, they watch "really very little television" because they're both working until bedtime.

"I'm working, and he has a pile of papers he has to go through for the next day." They also read for pleasure. "He's a big reader, always has been. Right now he's reading a book about Ike ('The Eisenhower Diaries')."

He doesn't bring his troubles as president home with him, she says.

"No, no, no, no. He doesn't come home and say, 'Oh, you know what happened today?' Never does that. I'll ask him what kind of a day he had. Maybe he'll go into some detail, maybe he won't. Maybe he just doesn't want to talk about it. You know not to press."

There isn't much she doesn't know about Reagan, but there isn't much she tells. Right now: How he's "maybe a little tight-lipped" if he's a little annoyed; how he sometimes throws his glasses if he's really annoyed; how he'll never tell her if he's worried, though she knows anyway because he may be a little quieter.

"I think it's his not wanting to put his worry onto you. He feels it's something he has to handle and cope with himself -- my son (Ron) is exactly the same way," she says.

What she thinks history will remember him for, and what she admires most about him, is integrity.

"People may disagree with him politically, or whatever, but I don't think anybody can ever question his integrity, his honesty," she says. "And I think that's what the people feel -- that he's not doing something for political gain but he honestly believes it. They have the feeling that he's sincere, honest, a good man."

She doesn't think Ronald Reagan has aged the way other presidents have, and she says she thinks that's because "he's very much at peace with himself, and very centered. There's no dark side."

But like her, he has a private side. Is there a private Ronald Reagan even she can't reach? "Oh, no," she says immediately.

She's standing now, ready to go back upstairs.

"Everybody should have a Ronald Reagan in their lives," she says, then in a throaty, almost sexy voice, she growls: "But I got him." By now, she's laughing uproariously, that musical laugh of hers that races up and down the scale.

"Nobody else can have him," she says. "So go away."