Tomorrow is William Schuman's 70th birthday. This week finds him at the Aspen Festival in Colorado, both to be honored with performances of his music and to address -- and work with -- the young musicians participating in the Festival School. It is typical of Schuman to celebrate his birthday working with the young. That always has been an important part of his life, and may in part account for his own extraordinary youthfulness at an age usually acknowledged with the adjectives "revered" and "venerable."

Few figures in American music (or European music, for that matter) have been so remarkably productive in so many areas as Schuman. He has shown himself to be an outstanding administrator, educator and general gadfly in matters musical, as well as one of the most eloquent and distinctive composers this country has produced -- and one of that rare species, a major American symphonist.

In Schuman's case, the production was entirely American, including performing experience in jazz bands and night-club dance bands and an early period of collaboration with lyricist Frank Loesser, with no study abroad except the single summer of his 25th year, spent at the Mozarteum in Salzburg. The following summer (1936), after his first year on the faculty of Sarah Lawrence College, Schuman began studying with Roy Harris, and in October of the same year he heard his own Symphony No. 1 played at a WPA concert in New York. A Second Symphony followed in less than two years and, at Aaron Copland's suggestion, it was performed by Koussevitzky in Boston.

Neither of those early symphonies pleased Schuman, who withdrew both shortly after the respective premieres and now speaks of himself as "the composer of eight-syphonies, numbered three through ten." It was the Third, introduced by Koussevitzky in October 1941, that brought instantaneous recognition and profound respect for Schuman as a symphonist, and as a major factor in American music.

After a full decade at Sarah Lawrence, Schuman became, in 1945, both president of the Juilliard School of Music and director of publication for G. Schrimer, Inc. In the latter post, which he held for seven years, he provided encouragement and guidance to many young composers; in the former, which he retained until he was named president of Lincoln Center for the Performing Arts in 1962, one of his several notable acts was the creation of the Juilliard String Quartet.

Since his resignation from the Lincoln Center presidency in 1969, Schuman has devoted himself to "writing music and eleemosynary projects," serving as chairman of the MacDowell Colony in Peterborough, N.H., as a director of the Koussevitzky and Naumburg Foundations, and in too many significant projects to catalogue here. He always has been an all-out activist for American music and American musicians.

For the Bicentennial year, Schuman created a radio series in which he talked about American music and played recordings of works in virtually every form. Of the 20 programs (in which only two or three of his own works were presented, and each of them brief), five were devoted to the American musical theater. Less than two months ago he delivered a hard-hitting paper during the annual conference of the American Symphony Orchestra League on the neglect of American composers in our concert programs.

He does all these things because, as he put it long ago, "If I did nothing but compose, I would go crazy and my publisher would go broke!" Early in his creative life, when he received his first Guggenheim fellowship, he asked for permission to teach two hours a week during the year it covered, even if his stipend had to be reduced, because "I just could not exist as a composer only."

With all his projects, Schuman has maintained a remarkable level of productivity, continuing to grow and, happily, never without recognition in either "official" gestures or public acceptance. His 1943 Whitman cantata, "A Free Song," was the first musical composition to win a Pulitzer Prize; his "Credendum," composed for the U.S. National Commission for UNESCO in 1955, was apparently the first symphonic work ever commissioned by the U.S. government (the State Department).

Schuman likes to describe himself as a romantic, frequently appending the adjective "unabashed." His peculiarly American kind of romanticism finds expresssion in his response to the words of Walt Whitman and the music of Charles Ives and William Billings (his "New England Triptych," based on Billings hymns, and his orchestral setting of Ives's Variations on "America" are "household-word" items on orchestral programs), to the popular verse of Ernest L. Thayer (the baseball opera "The Mighty Casey" and a subsequent cantata version, "Casey at the Bat") and the Sears Roebuck Catalogue (the 1897 edition of which provided texts for "Mail Order Madrigals" in 1971).

It is a romanticism characterized by the deeply felt, open-hearted compassion of "The Young Dead Soldiers" (a setting of Archibald MacLeish's poem, for soprano, horn and chamber orchestra) and the Ninth Symphony ("Le Fosse Ardeatine" , a memorial to Italian civilians massacred by the Nazis in World War II), in the unpretentious exuberance of the "American Festival Overture" (which preceded the Third Symphony by two years), the power and lyricism of the Tenth Symphony ("American Muse," the title Schuman also used for his radio series), the warmth and imaginativeness of the recent Concerto on Old English Rounds for viola, women's chorus and orchestra, and the soaring affirmation of the "Credendum."

"Unabashed," as Schuman has shown, need not be equated with "self-indulgent." His style can be expansive but never rambling, concise but not terse, often big as life but never larger than life, and always informed with credible vitality. It is his own character, rather than any striving for "local color," that accounts for the American texture of his music -- and it is recognized as such everywhere. He is not a folkist, and has only infrequently made use of themes not of his own invention; his work stands, rather, as confirmation of Copland's frequently expressed observation that "American music" would define itself simply as music written by Americans. There could be no mistaking Schuman's for anything else.

Back in 1940, barely a year after its Boston premiere, the "American Festival Overture" was not only performed by the NSO under Hans Kindler, but became one of the first works to be recorded by the orchestra. Last September Mstislav Rostropovich opened his third season as NSO music director with that work, and Schuman, who was present, was thrilled by the power of that performance.

Howard Mitchell conducted Schuman's music, and took the splendid ballet score "Judith" on the NSO's European tour in 1967. But it was during Antal Dorati's regime that Schuman's name became most closely associated with the orchestra's own. "A Free Song" was the concluding work (and the only American work) in the inaugural concert of the Kennedy Center Concert Hall, Sept. 9, 1971. Toward the end of the 1974-75 season Dorati performed and recorded the "New England Triptych," and a few months later he opened the Bicentennial season with a stunning program made up of three great American "Third Symphonies" -- those of Schuman Roy Harris and Copland -- with all the composers present. Later that season Dorati celebrated his own 70th birthday, in April 1976, by presenting an all-Schuman program comprising the world premieres of "The Young Dead Soldiers," "Casey at the Bat" and the Tenth Symphony.

Schuman is at work now on a major work commissioned by Leonard Slatkin and the Saint Louis Symphony Orchestra. We can look forward to hearing that and more new music of his productive 70s in Washington. In the meantime, the celebration of William Schuman's 70th birthday is in a very real sense a celebration of American music.