Medical science is "on the verge" of extending the average human life span to 85 years, substantially beyond today's American average of just under 70 for men and 77 for women.

The years up to 85, moreover, should mainly be healthy.

Age 85 will present a "rather abrupt" wall, however, since science, no matter how it tries, may never be able to extend average life beyond that.

This forecast was made to the American Association for the Advancement of Science here by two Stanford University medical professors, authors of a new study of how humans age and how aging is already being drastically affected by modern medicine. In a scientific paper and in a new book, "Vitality and Aging," Drs. Donald Fries and Lawrence Crapo make two main points:

Diseases can be eliminated, but the human life span seems fixed.

That span seems to be about 97 years at its ultimate. Man in fact might have reached that limit 100,000 years ago. The oldest documented human age is 115 years, and claims of great longevity among various remote peoples "have now been consistently shown" to be exaggeration.

There are many instances of life up to about age 97. Human organs, however, begin deteriorating at a slow but constant rate from about age 30.

"And eventually," these doctors say, "they fail lethally in all of us at about the same age."

In practical terms, they say, once a larger number of avoidable diseases and accidents is eliminated, 99 percent of Americans will die "of old age" between 73 and 97, with age 85 the average.

Why do these doctors believe the older years will increasingly be healthier?

To a large extent, they maintain, "the medical and social tasks of eliminating premature death are largely accomplished." One major killer, smallpox, has been completely eliminated. The leading killer of 1900, tuberculosis, has declined by 99.5 percent in this country.

Other major diseases are bowing to medical progress. Heart disease has been yielding rapidly just in the past decade. Several forms of cancer have begun to yield.

Already the average female life span is only about eight years short of the theoretical 85. Four of these years are accounted for by violent deaths. The life spans of both sexes will continue to lengthen until 85 is reached.

Even though human organs inevitably deteriorate, these doctors add, it is still possible to increase physical efficiency throughout life. This, too, is happening today, with an increasing emphasis on a healthier diet and exercise and avoidance of that leading killer, tobacco.

For these reasons and others, they say, "we are on the verge" in the United States of becoming a society "in which nearly all individuals survive" in a healthier state to advanced age, "and then succumb...over a narrow age range."

It is not impossible, they say, that average age will someday be extended well beyond 85, perhaps by genetic engineering or methods now unforeseen.

But such solutions, they say, would require "a quantum jump in scientific knowledge," and they suspect that either "super-longevity" or immortality will be at least as hard to achieve as the science fiction feat of traveling backward or forward through time.

The main lesson for society in changing patterns of aging? These doctors say that we need to start thinking about more ways to help the healthy aged lead independent lives, rather than just thinking about their medical care.