It is an unlikely scenario: Peter Pan grows up and becomes a Grand Old Man.

But no more improbable than our waking up one day this week to discover that Leonard Bernstein, the perennial wonder boy of American music, is a white-maned eminence in the tradition of such hardy elders of the baton as Leopold Stokowski, Arturo Toscanini, Pierre Monteux and Bruno Walter.Orchestra-conducting is unusual among human occupations in that it not only seems to promote long life, but also lends an increasingly saintly aura to the aging musician.

Bernstein, whose 70th birthday will be celebrated on Thursday, with Beverly Sills hosting a gala performance by the Boston Symphony at Tanglewood, has reached the enviable time of a conductor's life when he can scarcely sin, musically.

For him, the Age of Anxiety should be over.

People who formerly jeered or smiled indulgently at Bernstein's exuberance and choreographic antics on the podium, for instance, now greet his superheated performances as the ripened interpretations of a revered master.

Doubts, published or private, as to the positive effect of his raptures on orchestra players have all but vanished.

It is now generally agreed that his exhorting glances and dramatic gestures are not only effective showmanship, but the necessary eruptions of an overpoweringly musical nature.

Even his easily parodied foibles, such as his efforts to embrace every musician within reach at the end of a performance, can be said to represent the artist truly:

Bernstein would love nothing more than to put his arms around the whole musical world.

Lately, the whole musical world seems willing to reciprocate.

The Tanglewood gala, the first of four programs paying tribute to his career, is expected to bring together artists as diverse as Christa Ludwig and Kitty Carlisle Hart, Hildegard Behrens and Lauren Bacall, Van Cliburn and Roddy McDowall, Yo-Yo Ma and Adolph Green, Mstislav Rostropovich and Phyllis Newman.

Bernstein himself will conduct the last program on Sunday evening.

Though age has somewhat tempered his stage excesses, Bernstein remains a perplexing artist whom many people find hard to take, musically and personally.

No matter.

Orchestra musicians who earlier in his career condescended to him as "show-biz Lenny" now play for him with an ardor that few of his conducting colleagues can command.

And audiences, who made an overnight idol of him on that famous day in 1943 when he substituted as conductor of the New York Philharmonic, have always been in his pocket.

To a hero-hungry public, Bernstein was never merely a conductor or a composer or a pianist.

He was, from the beginning, America's musical celebrity.

He has thrown himself at so many different projects, and with such relentless expenditures of energy, that the trip from young prodigy to old master had to be pothole-strewn at times.

The wonder is that the career has developed as smoothly as it has.

His untamable ego, his unashamed need for attention and affection and his theatrical flair make him a satirist's dream.

Bernstein has always been an easy target for extramusical barbs - Tom Wolfe, you recall, pasted the "radical chic" label on him for his political activities involving the Black Panthers.

Was he a Renaissance man, professional head-shakers asked, or simply a restless jack-of-all-trades?

Here, after all, was a virtuoso pianist who found it necessary to become a symphony conductor, an electrifying symphony conductor who could not resist becoming a music-appreciation teacher on television, an acclaimed composer of concert works who also had to be the toast of Broadway.

The career has been a maze of such seeming contradictions. Consider, for instance, that this famously mobile conductor - of whom Harold Schonberg wrote in the early 1960s that he "rose vertically, `a la Nijinsky, and hovered there a good 15 seconds by the clock" - learned his craft, not under a similarly flamboyant maestro, but from the late Fritz Reiner, stern proponent of podium minimalism and the vest-pocket beat.

Like the composer with whom he most closely identifies, Gustav Mahler, Bernstein has wrestled during much of his career with conflicting impulses.

It was his ability to empathize with Mahler's psychological torments, as expressed in searing performances and recordings during the 1960s, that persuaded the music world once and for all of Bernstein's importance as an interpreter.

However, Mahler's was only one of the ghosts living under Bernstein's artistic skin.

Apparently there was an Offenbach hiding there, too, prodding him to compose such relatively light musicals as "On the Town," "Wonderful Town" and "West Side Story," as well as the more ambitious operetta "Candide."

Like Offenbach, whose genius embraced both the high-spirited "Orpheus in Hades" and the grandly operatic "Tales of Hoffmann," Bernstein has successfully worked both sides of the esthetic street.

He could move freely from the popular raffishness of the ballet "Fancy Free" to the heavier pretensions of his symphony "The Age of Anxiety," based on an Auden poem, or his "Serenade," after Plato's "Symposium."

In him, populism and elitism have always coexisted, however uneasily.

It does seem reasonable that a former wonder boy should write incidental music for J.M. Barrie's "Peter Pan," though not inevitable that the same artist should be attracted to the Lamentations of Jeremiah as the basis for his Symphony No. 1.

He can produce a "Kaddish" Symphony inspired by Hebrew liturgy as well as a "Mass" that refers at least nominally to Roman Catholic rites.

Wearing one hat, he composes "Prelude, Fugue and Riffs" for jazz ensemble, and, under another, "Chichester Psalms" on biblical themes.

He has been at home both lecturing at Harvard on Noam Chomsky's linguistic theories, and on national television, where his music-appreciation programs, begun in 1954, ran for 15 years.

He writes light verse and sets Rilke poetry to music.

He is - one more apparent conflict - an intellectual striver with a yearning to be respected in bookish circles who nonetheless swims against academic tides.

Not the least of his contributions to 20th-century music may turn out to be his firm refusal to bow to the peer pressure that in the post-World War II years sent so many of his colleagues scurrying after Webern and, finally, Stravinsky, into the Serial camp.

While music director of the Philharmonic, he remained a tonalist in his own compositions but made sure that audiences knew what was going on in avant-garde circles.

Playing both advocate and skeptic, he conducted more than 40 premieres of atonal works, peppering his programs with names such as Carter, Babbitt, Cage, Berio and Stockhausen.

But, as he did not hear atonal music ringing in his own head, he chose to be unfashionable.

Critics used to grieve over Bernstein's unwillingness to settle for only one career in a lifetime.

Detractors joked that he could not decide whether to be the next Toscanini, the next Beethoven or the next Cole Porter.

He still has not decided, but we have learned to let him run his own confusing race.

It is a career with more heads than Hydra, who had nine of them, but no less fascinating for that.

A one-headed Leonard Bernstein would not be Leonard Bernstein, and American music would be markedly poorer than it is today.

Certainly more boring.