MICK JAGGER is back, prancing on the big screen in Hal Ashby's Let's Spend The Night Together, the concert film of the 1981 tour, which brought him and The Rolling Stones $60 million from North America's wallets in what was widely hailed as the band's goodbye to showbiz. A year later came The Who's final worldwide tour, the band's second in three years. There are persistent rumors that the Stones may tour again next year. So far in the 1980s, it seems, rock fans have been spending an inordinate amount of time listening to the aggressive death rattles of these two once great bands, who keep threatening and promising that they're finally going to call it a day. By now the question isn't why, but who really cares?

If all this swan-song overkill is tiresome. It does have some point: it is only possible because of a legitimate feeling from the rock audiences that this really could be the last time, that fans can grow too old to appreciate rock, and that musicians can get too old to play the music.

Next year, Diana Ross, Johnny Winter and Lou Reed will have something in common: they will all turn 40. Jagger hits 40 in July, and Bob Dylan, Joni Mitchell and Aretha Franklin have already passed that mark, which stands like the tape at the end of a race course.

The barriers against rocking on into middle age aren't physical - not the way age is a barrier in hockey. Instead, they're ideological. Rock wears the burden of being considered an exclusively youthful expression of exuberance, like writing lyric poetry or swallowing goldfish. Whatever twists and turns in style the music actually takes, there is one standard measure of what is considered "authentic" in rock: how well and clearly it represents the values of youth.

The popular argument was first set out in a 1969 book, Carl Betz's The Story Of Rock, which outlines the qualities that make rock exciting: spontaneity, simplicity, expressiveness and "relevance" to life experience. It should encourage a lack of self-consciousness, an intuitive response to experience and, finally, in the word that embraces the whole esthetic, it should be "youthful," with all the anarchy, gang instincts and sexual experimentation that the adjective implies.

Rock musicians are haunted by age, not only by looking in the mirror and watching their physical decline but also by seeing themselves grow away from the record-buying community the imagine they imagine they represented. Getting old hurts both their careers and personal pride. Pete Townshend, who made a mission out of serving "the kids" in his audience, admitted by the mid-seventies: "I really hate to be too old to be doing what I'm doing." Mick Jagger's comments on age echo Townshend's self-doubts: "Quite honestly," he said, "once you are able to reproduce, you are over the hill. You start to go downhill at 18, physically."

One of the bands that was most committed to its youth audience in the late seventies was The Jam. The band decided to quit rather than to grow old and stale in public. The Jam's contemporary, Elvis Costello, said he'd prefer to be run over by a bus than get old: "I'm deadly serious about this. I'm not going to be around to witness my artistic decline." Of course, rock history is studded with young, celebrity deaths, with their messianic overtones; to die young and stay pretty has become part of the rock star mythology.

Some with a healthier sense of self-preservation tend to put rock behind them as one of their childish toys. For John Lennon, The Beatles were like "a gang of boys," a stage to be outgrown before he settled down to matrimonial duty and responsibility. Dozens of artists, from Elvis Presley to Paul Anka, have been anxious to parlay a talent for exciting juveniles into something more substantial: Vegas, where their adult music provides an aural backdrop to the jangle of slot machines.

Almost by definition, an old rocker is just the shadow of the real thing: mass artists are attached to the dreams and aspirations of a mass audience, which is fervently fixed on a youth ideal. The trouble is, that youth-rock community has always been an inspirational myth. The power of this myth, perpetrated by the sense of community that AM radio provides, is now recognized as a pleasant illusion. English rock critic and sociologist Simon Frith wrote in a recent essay on rock's evolution: "The rock 'community' refers not to an institution, not to a set of people, but to a sensation ... The rock 'n' roll experience depended on a mutually nourishing relationship between the audience and the musicians, but not on shared lives. Rock 'n' roll singers were not folk, neither were they just symbols of achievement, punk signs of what anyone could do." Rock albums and radio can give listeners a sense of community with their fellow fans, and that search for the illusory community of shared values and needs continues to be one of the most important motivating forces in pop culture.

Since 1970, the market share of the music industry which is known as the rock community has splintered into several dozen "lifestyle" markets, all of which were reflected in different musical styles of the last decade. Today, the average American is over 30 and, from the early seventies on, most successful pop music - progressive rock, fusion, country rock, disco and the singer-songwriter - has been an attempt to fuse youthful music to adult requirements. The only unqualifiable youthful movement in music of the decade was the rise of heavy metal, one youth trend few adults wanted any part of.

The rock and roll revival shows, which began at the tail of the sixties - and, in a sense are still continuing with last hurrahs of The Who and The Stones - were generally treated with contempt by the rock press, but they have had an enormous effect. Such acts as Chuck Berry, the beach Boys and Bo Diddley all found themselves with new careers after 1969, filling grandstands year after year to apparently younger and younger audiences. The rock revivals showed that music liked by young people is not tied to the age ot the performer. On the negative side, it made contemporary rock seemed buried in lethargy, making it little more than a pale reflection of its own past.

Complicating the rhetoric of rock-as-youth-culture was the change in rock in the mid-seventies, initiated by The Ramones in America and The Sex Pistols in England. Like most things in rock music, punk - and it's follow-up style, new wave - were orchestrated by adults for a youth market. Such new wave idols as Iggy Pop, Patti Smith, Deborah Harry and The Police were well into their thirties, but new wave was clearly much more a cult of sensibility than age, built around a mid-sixties' idea that rock could be wedded to other avant-garde art forms. Almost immediately though, the musical position became reactionary, and media and record companies fell back on the traditional language to explain a complex new phenomenon: "Take out the word, Sex Pistols, and substitute the word, Rolling stones." Pointed out Stones' guitarist Keith Richards when The Sex Pistols first arrived, "and you could be reading the same press that was written about us in 1964."

Seven years after punk, the new English pop groups, from Human League to ABC to Culture Club, have managed to appeal to both the younger and the older generations of rock fan: they look avant-garde for their young fans and sound enough like traditional pop music for the older fans, and the idea of "new music" has been misused as some sort of approval of youth in general. When Clive Davis, president of Arista Records, called for more new music on the radio, he proclaimed, "Youth must nog be disenfranchised any longer."

The experts know better: radio programming is not a question of age or youth, but of the most commercially safe way to hold the largest possible audience. A couple of week ago, Billboard reported that the most influential radio consultant in America, Lee Abrams (the man who invented the Superstars format which determined the hard rock sound that has dominated rock radio for the past three years) announced to a group of radio programmers the time had come for a musical revolution. New music will now be played extensively on all his stations. Culture Club, Men at Work, The Psychedelic Furs and Thomas Dolby will replace Led Zeppelin, Thin Lizzy and REO Speedwagon. Abrams, incidentally, didn't see any change in the basic rock audience - 18- to 24-year old males - but he did see the new music appealing more to the 25- to 34-year old group and less to the teens. "We want to avoid too many teens," Abrams warned his radio stations, "so we have to be really selective of our music choices."

What the new wave shift in music may have accomplished is to undermine the assumption that rock's commercial success and esthetic appeal were inextricably bound together. Fans have been educated to look toward the fringes of rock for satisfaction, to take a more active approach to discovering the music they want to hear. One of the most intellectually attractive ideas about rock is not its youthfulness, but the fact that it allows a mass audience to experience vicariously social alternatives to the middle-class norm - an imaginary community, if you like, of bohemian or counter-culture values. Rock still has the potential to evoke, as it has in the past, the values and insights of minority cultures - hillbilly, black, gay or bohemian. In this area at least, some aging rockers are still leading the way.

Lou Reed, whose band the Velvet Underground was prophetic of punk, has always dissociated himself from the popular music of his time, and his work these days is nog exception. Last year, he released The Blue Mask, an album that dealt with mythology, violence, imagination and domestic life. In its consciously ingenious way, The Blue Mask may be revolutionary - the first genuinely artistic middle-aged rock album.

Two more examples: Captain Beefheart, whose real name is Don Van Vliet, is a veteran of several movements and has remained more eccentric and daring with his experiments than most of the new music experimentalists. Recently, the videotape accompanying his album, Ice Cream For Crows, was judged worthy of inclusion in the Museum of Modern Art's permanent collection. It was not judged worthy of inclusion on the 24-hour rock video station, MTW: at 42, Van Vliet was judged to be too old. Randy Newman has a curmudgeonly stance that is much better suited to him at 39 than it did when he was in his twenties.

Songwriters such as Newman, Beefheart and Reed and maybe even Bob Dylan (once he figures out how to express the spiritual condition of un-Born Againism) can offer insights, angles and shades of experience that younger musicians can't yet attain, and the dictates of radio formats or advertisers should never be allowed to hide that. Everyone who has grown up with rock music knows its strengths are its sense of play and youthfulness, but interest in the music has nothing to do with chronological age and everything to do with mental vigor. Besides, there's a whole generation out there begging to know the answer: how does it feel to have a mid-life crisis with a backbeat you just can't lose?