IF MEDICAL science ever concocts a cure for cancer, our young playwrights could find themselves in a heap of trouble.

This sounds like a deplorable generalization, but consider the evidence. If you X-ray the last several theater seasons here and in New York, you will find incurable malignancies lurking in hit after box-office hit (not to mention flop after flop).

Key figures in Peter Nichols' "The National Health" and Bernard Slade's "Tribute" have the disease, as do the two principals in Ronald Ibman's "Cold Storage" and no fewer than three characters in Michael Cristofer's "The Shadow Box".

Of course, some of these dramatists might have been able to drape their tales around a subsitute malady like Legionnaire's Disease or prickly heat, but it would not quite be the same.

Cancer "is understood as mysterious, a disease with multiple causes, internal as well as external," says Susan Sontag in "Illness as Metaphor." It is "the disease that doesn't knock before it enters" and it is widely identified with affluence, industrialization and hyperactivity.

Not that other ailments have been ignored in this mad rush to turn theaters into rest homes and hospices. As the box scores on page G10 should demonstrate, retirement, euthanasia, disease and death have suddenly become the indispensable subject matter of the English language drama.

Washingtonians have been able to inspect the trend at close range in Brian Clarkhs "Whose Life Is It, Anyway? (about an auto-accident victim demanding his disconnection from life support), Arthur Kopit's "Wings" (about a stroke victim struggling to control her powers of speech) and Ernest Thompson's "On Golden Pond" (about an elderly couple sitting inactivity and mental and physical decay).

And we are about to be visited by D. L. Coburn's "The Gin Game (in which Hume Cronyn and Jessica Tandy are confined to a nursing home) and Steve Carter's "Nevis Mountain Dew" (in which Graham Brown plays a polio victim confined to an iron lung). "The Gin Game " opens Wednesday night at the Eisenhower Theater and "Nevis Mountain Dew" April 25 at the Kreeger.

Demography is certainly on the side of these morbid preoccupations. The number of Americans over 65 has nearly doubled in the last 30 years to a figure approaching 25 million. Our median age, which was 27 in the mid-1960s, will reach 35 around the turn of the century. Already, the American male who survives to age 65 can expect to live another 14 years and the American female another 18 years. And about 30 percent of the federal budget now goes into programs addressed to the elderly and the retired.

But a look inside the theaters hosting some of these stories of infirmity suggests that something more important may be at work than a simple effort to keep up with population shifts. The actors onstage are older than the ones we may used to, but the audiences and the authors are conspicuously younger.

Have you invented a new rite? Are we using the theater to confess (as swiftly and painlessly as possible) a litany of sins committed against our elders-which is to say parents, grandparents, aunts, uncles, co-workers and total strangers? It looks that way.

"I'm no one's emergency!" declares Dorothy Loudon as the widowed mother in "Ballroom" (Michael Bennett's just-closed $1.5-plus-million musical), when her children and in-laws come rushing over to her apartment one night because she has been out late. Elsewhere in the play, Loudon announces that she has a date. "Another one!?" comments d meddlesome relative. "How many am I allowed?" she replies drily. Both these proclamations of gray rights (actually, Loudon's hair verges on peach color) are greeted by thunderous, cathartic applause.

In "Ballroom," "On Golden Pond" and "My Old Friends" (a hit off-Broadway musical set in a rest home), the ill or aged protagonists let us know they have had it up to here with the next generation, including their kids and doctors. They are tired of lies and condescension and even solicitude unaccompanied by honest interest.

For half a century or so, Americans have spent ever larger sums of tax money on medical and pension programs that have enabled us, as individuals, to ignore these troublesome people. The effort has backfired: With our money (and their own) Americahs old and sick are raising hell and demanding our attention. In effect, they have filed a class-action negligence suit against the young and middle-aged. They have simply hired playwrights instead of lawyers to draft it.

Guilt, of course, is not the only motive that makes us susceptible. Old age is a subject with wide intrinsic appeal. "Very few of us are completely removed from the situation," D. L. Coburn points out, "and we're all potentially in it."

Painters and photographers came sooner than playwrights to an appreciation of beauty in age. Yet we tend to examine their art in a more detached and transitory manner. Plays argue. They won't leave us alone. And sometimes they keep tugging away even after the curtain has plunged.*t"my mind and body are running a great race to see who can poop out first," says the septagenarian hero of "On Golden Pond." Tom Aldredge, who plays the part, has gotten himself up to look like a stuffed trout with lower back pain, but he, too, strikes some telling blows for grandparental emanciaption.

Ernest Thompson's paly, which passed through Washington this winter on a circuitous passage from off-Broadway to Broadway, has a wit and complexity rarely associated with the elderly in theater, film or fiction. He has refused to give us the same mushy, idealized elders and ogre-ish youngsters who populate several of the other plays of the ilk, and he has permitted his old folks to like the young-if reluctantly-as well as lecture them.

At the other end of the spectrum are such bargain-fare guilt trips as "My Old Friends," in which two rest home residents fall in love and (because there was nothing terribly wrong with their health to begin with) cut loose for the outside world.*tHere, malevolent middle age is portrayed by a loudspeaker that constantly barks orders at the cowed inmates about thins they are forbidden to do or touch. The moment of truth comes when the hero-again to a vast ovation-illegally unlocks a piano that is supposed to be played only at Christmastime.

"My Old Friends" also offers a white-haired, freshly unretired jazz singer of the 1930s, named Maxine Sullivan, the chance to sing a song called "There's Still a Little Spark Left in the Old Girl Yet"-a kind of senior citizens' anthem. Like "Ballroom," this likable but thoroughly simple-minded show went out and recruited some has-been-and-still-are show people who had given up the stage forever, they thought, in eras that had no use for them.

These one-note odes to old age will probably not be with us long, and the same goes for the theater of terminality representative by "Cold Storage", "Whose Life Is It, Anyway?" and "The Shadow Box."

But "On Golden Pond," "The Gin Game" and Hugh Leonard's father-son play "Da" offer hope that the elderly may eventually emerge from a period of awkard over-attention to assume something like their rightful place in the theater.