IT WOULD be easy to bill Michael Weller as the theatrical spokesman for a generation. His "Moonchildren" is one of the most perceptive accounts of collegiate life in the mid-'60s. His "Loose Ends," which opens Wednesday at Arena Stage, takes the temperature of the same generation one decade later. And he wrote the screenplay for "Hair," Milose Forman's film version of the original rock musical, scheduled to be released next month.

But Weller -- who is 36, after all -- refuses to represent The Young or even the older brothers and sisters of The Young. "I don't want to summarize my generation," he pleads, turning aside all requests for pontifications about his contemporaries. "Whenever I feel myself sneaking some statement into my work, I cut it out. This prevents some people from realizing you're serious, but I desperately do it on purpose. You just have to be intrigued by your characters, and let them have their head. Within reason."

He has given his characters in "Loose Ends" their head.They drift through nearly an entire decade in eight scenes, falling in and out of love and bed, changing their ambitions and dreams with each passing year.

Weller denies that it's story about himself or any of his friends in particular. But he has been at loose ends himself. Trained as a classical composer, he never composes. Raised as an American, he spent the period from 1965 to 1973 in England, where "I learned my (play-writing) chops" and where "I got my excitement about America." He has been married and divorced. He knows a thing or two about ambivalence.

Among his "split feeling," he says, "I have this very American belief that people can change, that possibilities are infintite, that the will matters. But then the other side of me slips into Eurpoean gear and I wonder why can't I accept things and get on with it."

The former view was the one that dominated the original "Hair," in which the Age of Aquarius was dawning and all we had to do was let the sunshine in. "Hair" has been labeled naive, but Weller says the film version of "Hair," while retaining the "exuberant spirit" of the original, will not be naive. "We're not naive," he says, referring to himself and Forman, "and if we did something that looked that way, we would stop it."

Weller didn't remember "Hair" when he was asked to write the film. "I had seen it only once with my dad, who called it the hippie 'South Pacific,'" he recalls."I liked the spirit and invention, but it didn't seem to me to be about people." An acquaintance of Weller's who had seen the original "Hair" 18 times was not even ablt to remember the details of the plot when Weller asked.

He wasn't sure he wanted the job until he was told Forman would direct.Forman's "Firemen's Ball" was the only filrm that had ever inspired Weller to want to write for a film director, he says, "and I didn't care if it was the telephone directing."

Weller had never written a produced script, so it may seem odd that he was asked in the first place. But Peter ("Equus") Shaffer put in a good word for him with producer Lester Persky, Forman liked "Moonchildren," and Weller says he knew the language of the era, if nothing else.

Though he has spent the "Hair" years in England, Weller had his own confrontation with the draft there, much like the "Hair" hero did. Called for a physical at an American air base in Emgland, Weller shoped up with a hangover, he recalls. He appeared unduly depressed to one of the sergeants, who referred him to a psychiatrist. "This shrink was sitting in this oak-paneled office, surrounded by books of Voltaire and the Yale alumni magazine and miniskirted girls serving tea. It seemed like a good place to act weird. He said, 'I get the feeling you're trying to psych out,'" says Weller. "I said, 'Why not?' and he let me go. He was a nice guy, and very cool, sitting the war out in England. But I also heard him do the opposite to this bomber pilot who had freaked out. He was telling the pilot he should go back."

Experiences like this are probably helpful to the screenwriter of "Hair," but Weller did not feel ideally suited for the job. He says Forman asked him what his ideas were for the script. Weller replied that he had none. That's fine, responded Forman -- no preconceptions.

Weller won't talk about what he and Forman did to give the sprawling "Hair" book a shape. He wants people to be surprised when they see it in the theather. But here are a few clues: "Hippies were a press invention. The show counts for its effects on a sentimental attachment to these people -- that's what's dated about it -- but actually there were very few of what were called hippies. It was like a new product that made people vicariously feel they were part of what was happening." To concentrate on their outre behavior now, says Weller, would be "like doing a Prohibition musical and expecting everyone to dig the drinking."

Does this mean that all that countercultural tumult was for nothing?

"Generally, the world is the way it is and what changes are the way people see it," says Weller in his "European" gear. But then, shifting back to his "American" gear, he notes that "certain notions of personal freedom" have expanded since the'60s, "for better or worse." And he adds that the apparent declining literacy of children may indicate "the truth about how we're changing back to an oral and visual way of expressing ourselves," after several centuries of print domination.

"I do feel something incredible is going on," says Weller. And when he writes his plays, it's like "taking notes." But, as if sensing the danger of creeping pontification, he warns that "there is such uncertainty. I feel no ruge to explain it. I just want to report it."

After seeing Weller's play "Fishing" at the New York Pubic Theather several years ago, a friend told him he should write about subjects closer to his audiences, such as "how to make a relationship last." This marked the genesis of "Loose Ends."

At first Weller set his story in the last year of his couple's relationship. But he changed his structure after David Hare, a British playwright who was visiting Weller, showed him the script for his "Plenty," a play staged by the British National Theatre last year. From "Plenty," Weller borrowed the device of moving his play forward over a period of years, leaving "mysterious spaces between the scenes" during which the audience could use its imagination to figure out what was happening in the meantime.

Perhaps "Hair" influenced this decision too, he acknowledges. He learned how much fun it is to change his locations as in a movie. For reasons known only to him, he also changed the name of his central female character from Carol to Susan. They are "completely opposite names," he says.

When Alan Schneider of the Juilliard School requested a one-act play for his students, Weller wrote an epilogue to "Loose Ends." Juilliard never got around to doing it, but finally it was staged in New York last year as "Split," and the Reception it got encouraged Weller to finish "Loose Ends." The writing of a play was therapeutic after "Hair," says Weller, "It was very good to be back where it didn't cost a million dollars a day." And after years of "stewing for a long time" about the play, the writing came fast.

"I'm fanatical about getting the form right, and then I work intuitively," he says. "I dream about people I know, and it's like a pencil sketch in the brain. I like to have all the elements in mind so I can write in a kind of trance." He reads the results aloud to his girlfriend before sending it off and occasionally changes words that don't sound right.

David Chambers of Arena Stage snapped up "Loose Ends" after one reading, and Schneider was recruited to direct. The Weller-Schneider duo gave Arena one of its finer monents in 1971 with the American premiere of "Moonchildren," and Weller likes the Arena stage for the way "it isolates actors like a microscope." After "Loose Ends," Weller will return to one of the more formidable screenwriting tasks of the decade: E.C. Doctorow's "Regtime," again for Milos Forman.

Weller says he likes to "jam a play full of stuff so it's spilling over the edges." But he also writes "10 times less than I should. I only feel comfortable when I underwrite. It forces the audience to listen very carefully, and you can very sparely suggest a lot of things."

However, the "much more severe" world of Pinter and Mamet is not for him. "I would feel I was describing a world that's too controlled," he says. "I like to write about people who are living by accidnet."