Octogenarian Malcolm Cowley challenges some old-age stereotypes; The View from 80, by Malcolm Cowley. New York: The Viking Press. $6.95.

"One's 80th birthday is a time for thinking about the future, not the past," writes Malolm Cowley in this graceful, wide-ranging essay. Like a vigilant sea captain, he scans the horizon of the elderly for storms, shoals, and signs of fair sailing. His book will touch readers of any age.

Expanded from a 1978 Life magazine article, this 74-page volume offers a refreshing and candid discussion of what we call old age by a distinguished man of letters. Following hard on the heels of Cowley's 11th book, a memoir of the depression era entitled "The Dream of the Golden Mountains," published last spring, it offers ample proof that the 70s, 80s, and beyond needn't be years of drift and inactivity.

"The View from 80" doesn't urge every octogenarian to be as physically active as the 86-year-old Key West man who wrote to Cowley describing his morning regimen (20 situps, followed by a bicycle sprint to the beach for calisthenics, and a brisk swim).It does urge the elderly to live with purpose, strength, and vigor.

Cowley tries to avoid painting too glowing a picture of the "golden years." He believes most the literature on the subject, from Cicero to Alex Comfort, does just that, possibly because so many of the writers were mere "lads and lasses" of 50 or 60, trying to look cheeringly ahead, but writing without genuine knowledge or honesty. A "sincere report, not a mere self-consolation" (to use Cowley's words of praise for another writer) is the goal he must have set for this book.

Even though Cowley does not, some readers will challenge the idea that heredity and the genes determine one's life expectancy, reflected in the Census Bureau statistics he quotes. He does assault several other stereotypes, however , including senility, the supposedly limited potentials of the elderly, and the reasons for "growing old."

What does Cowley see on his horizon?

Forming part of the view from 80 are what he refers to as the "messages from the body that tell us we are old," and he devotes two or three pages to a description of what they sometimes have to say. But he cautions that the more insistent and devastating messages come from others, a warning both old and young can take to heart.

For instance, Cowley recalls the near-collision he had many years ago in a parking lot. The irate driver of the other car suddenly cooled down when he caught a glimpse of Cowley, dismissing him with the cutting words, "Why, you're an old man." He also remembers the time a few years later when a young woman got up and offered him her seat on a crowded bus. He declined. But when a similar offer was made the following year, he accepted, "though with a sense of having diminished myself."

"We start by growing old in other people's eyes, then slowly we come to share their judgment," he concludes, implying that the superficial judgment is more cruel than anger, even if meted out with kindness.

Cowley believes the greatest "temptation" of age is "simply giving up," and he decies society's readiness to furnish a compelling rationale for surrender: Too often the old "are made to feel that they no longer have a function in the community. Their families and neighbors don't ask them for advice, don't really listen when they speak, don't call on them for efforts.

"One notes that there are not a few recoveries from apparent senility when that situation changes," he continues. "If it doesn't change, old persons may decide that efforts are useless."

Though touching on public policy and retirement practices, "The View from 80" isn't written for policymakers.Instead Cowley hopes it "will serve as a personal message to each of my comrades in age."

What is the message?

Briefly, the physical problems of the elderly aren't as decisive as the mental qualities with which one responds to them. That the vices frequently ascribed to age -- avarice, untidiness, and vanity, among them -- should be guarded against. That our modern "customs or inadvertencies" in disposing of the aged are no more civilized than the practice of the Northern Ojibwas, who brained their elders with tomahawks during ceremonial dances.

Cowley believes that the thought of death, "never far absent," can serve more as a stimulus to vigorous living than as a threat. And he feels even more passionately that a sense of purpose, accompanied by some kind of work outlet, is just as essential later as it is earlier in life: "Poet or housewife, businessman or teacher, every old person needs a work project if he wants to keep himself more alive. It should be big enough to demand his best efforts, yet not so big as to dishearten him and let him fall back into apathy."

Though each individual must discover his own purpose, Cowley feels a special lure in trying to find a "shape or pattern" in life. He writes, "Our lives that seemed a random and monotonous series of incidents are something more than that; each of them has a plot."

This purpose attracts others. One 90-year-old woman wrote Cowley: "You have inspired me to begin writing a sort of memoir of my life -- something my elder daughter . . . has asked me many times to do. . . ."

I, for one, hope Cowley and his contemporaries will feel impelled to make their discoveries -- particularly of that enduring essence of the self that isn't diminished with age -- and will share them with us all.