The graceful aging of President Reagan's insides has become a matter of record, a happy aspect of an otherwise chilling episode.

The skeptics of both major political parties who cited Reagan's 70 years as an obstacle to his presidential capacity must now be silenced, if not forever, at least until he's 80 or 90.

Better even than that, believes Dr. Robert N. Butler, Reagan's resilency should end the often unconscious tendency to attach age to his every mention.

"The best thing about all of this," says Butler, who is director of the NIH National Institute on Aging, is that "It's being made very clear that this (Reagan's recovery) is not extraordinary.

"While I don't want to detract one iota from his strength and humor, it becomes a very important message to the American people that we've been perpetrating some quite inaccurate stereotypes about aging. There are a huge number of people out there between 70 and 80 who do extremely well. Good recoverability from a variety of surgical procedures, including ones involving the chest, are not surprising.

"The president," he says, "becomes a very important role model here." The questions raised are these: How is it that one person at 55 is ready for the porch rocker and another at 70 is back in harness, so to speak, not 24 hours after he is shot in the lung? What, moreover, can the rest of us do to enhance our own chances for such graceful physiological aging?

"A fair question," sighs Dr. Reubin Andres, clinical director of the National Institute on Aging and director the principal directors of the longitudinal study of aging, in progress since 1958 for men and 1978 for women. "I would say that we do have some information about this, but it is primitive compared to what needs to be knwon.

"First of all," he says, "it should be clear to you when you look at people who are 70 that there is a wide disparity. In fact, though, all 20-year-olds are not alike either.

"There are ways of estimating physiological or biological or functional age, whatever you call it, as distinct from chronological age, and if you measure the function of many different organ systems, you'll find great disparity in performance.

"Some organs may be functioning better than you might expect at a given age, and others not as well."

Geriatricians have found that there are some functions which do tend to deteriorate with age and others which were thought to do so, but probably do not. To make it even more complicated: the ones that are supposed to deteriorate do not in some people and the ones that aren't supposed to sometimes do in othes.

"Where our ignorance begins to show," says Andres, is in determining "what causes differential patterns of aging.

"There is no question that genetic makeup plays a role. I saw the president's older brother on TV and he's a terrific-looking specimen. Judging by appearances and speech, it is clear he has aged well.

"But while that's important to know about, it's nothing you can do anything about at the moment." (In the president's case, the genetic coding is uncertain. His father died of a heart ailment at age 56. His mother lived until the day after her 79th birthday.)

"And of course there is the element of luck," says Andres. "If you've been run over by a truck, it doesn't matter too much how long your parents lived or how you manage stress. . . .

"There are things one can do, though, and among the vaiables are these: nutritional habits, activity patterns, cigarette smoking, alcohol consumption, personality patterns, which include stress responses.

"And having ticked these off -- while we're reasonably certain those factors are important -- once you get down to details, it get stickier.

Nutrition: "We know a great deal about gross malnutrition (such as scurvy, beriberi or pellagra) but when you get to the more delicate details of what you should ingest in order to age gracefully, we have to beg off and say there really is no good evidence for what those patterns should be."

Even the role of obesity, says Andres, is somewhat controversial now. "It is generally agreed tht it's not good to be extremely lean or grossly obese, but just exactly where badness begins on the obesity scale is arguable."

Activity : "The information is just as vague as in nutrition. It is so reasonable that being vigorously active would improve one's health, you almost have to be a Scrooge or a curmudgeon to say, 'Okay, show me the evidence.' That is very hard to come by because you can't conduct the kind of experiment in humans that would permit you to say that it was physical activity that has kept you younger. It gets you into the chicken and the egg problem: Are older people who are vigorous and active that way because nature has given them good genes and permits vigor and activity? Or is it that people who have been vigorous and active reach old age in good shape?

"Most believe it is probably the former, believe it is good to conduct a life of reasonable vigor, but there is no real proof."

Smoking : "This is just plain bad news. It's bad for overall mortality, probably for general aging." (Butler notes that even after 40 years of smoking 2 1/2 packs a day, ex-smokers' lung capacity is virtually normal after a year of no smoking.)

Alcohol : "New studies indicate that the teetotaler has a shorter lifespan than mild drinkers, but once you get into heavier drinking, the mortality goes up again." (President Reagan's father was known as a hard drinker; the president drinks very little.)

Personality : Geriatricians believe that the absence of stress is as damaging as its poor handling, and that leadership is a great predictor of longevity.

Another geriatrician, Dr. Valery Portnoi at the George Washington University Medical Center, also sees in the shooting tragedy an opportunity. u"I would hope," says the Soviet refugee, "this occasion would be a kind of trigger to use the potential that is there."

It is Portnoi's belief, based on studies of cell replications, that humans are programmed to live 150 years, that there are "three clocks of aging: social, biological and psychological . . . but we live in an artificial structure that may not permit us to use our biological potential."

In his work in the U.S.S.R., in Israel and in this country, Portnoi has seen older people have memory lapses "because they believe they're supposed to have them.

"They do poorly on evaluations because they fear they are being tested for craziness and are afraid they will be deemed senile. Most of the tests are occupationally oriented. If you make the tests judgmental -- how to behave in this or that situation -- then the older people perform better than the younger."

It is true, he says, "Older people have a role to play as people who are wise , to help people to live, based on their vast experiences."

So far as the Aging Institute's Butler is concerned, the recipe for long life may not be proven, but strong evidence suggests this:

Stop smoking, develop a sense of humor, don't worry about stress (or, presumably, anything else), keep life full of complications and goal-oriented; and remember that "the 'Who's Who-sers' live longer."

And as for Shakespeare and his "as You Like It" seven-ages-of-man speech, ending up "sans teeth, sans eyes, sans taste, sans everything"? "Shakespeare," says Butler, "was dead wrong."