Probably it happened late in the 1960s, the decade during which Copland - as old as the century - began to withdraw from the arena of composition per se and took up a new career as cultural celebrity. At the time, his best work was hardly his most recent. The music for which he is famous today was also the music for which he was famous then. But these pieces - the ballets ''Billy the Kid,'' ''Rodeo'' and ''Appalachian Spring,'' the film scores for ''Of Mice and Men,'' ''Our Town'' and ''The Red Pony,'' the tone-poem ''Lincoln Portrait'' and the ''Fanfare for the Common Man'' - all date from the troubled years of World War II and its optimistic aftermath. As their titles suggest, they are distinctly American in their extra-musical subject matter.

Distinctly American, too, is their sound. At least, that's the way their sound came to be regarded by the vast audience who heard and almost immediately loved them. Whether or not Copland in fact makes use of materials from the repertoire of American folk music, the tunes from his heyday have an earthy quality about them that Americans like to think of as somehow of this land. His rhythms sooner or later turn propulsive and infectious, easy to grasp and ride with in much the same way that the rhythms of indigenous American pop music - ragtime, big-band swing, vintage rock 'n' roll - are irresistibly engaging. His trademark orchestration, with its wide spacing and emphasis of ''open'' intervals, can be heard as a metaphor for prairie vistas and mountainscapes; that they are indeed heard in that way is evidenced by the countless times they've been imitated in the soundtracks for Hollywood's Americana epics.

Copland was born almost 90 years ago in Brooklyn. That, of course, makes him an American composer, and even the pieces he wrote during his student years in Paris naturally count as American works. Still, there is something about his output from the late '30s and '40s that prompts one to use the adjective in a different sense. Perhaps less for what they are, musically, than for what they have come to represent in the minds of those who know them, ''Appalachian Spring'' and the like are symbolically American works. They are icons that have meaning far beyond their mere notes, emblems whose significance is fully apparent even when they're heard only in fragments. That Copland is their creator makes him an ''American'' composer in a class of his own.

''Copland has often been called the 'dean of American composers,' whatever that means,'' conductor Leonard Bernstein told the audience at a Young People's Concert given to celebrate Copland's 70th birthday. ''I guess it's supposed to mean that he's the oldest, but he isn't the oldest - he's just the best.''

The statement is debatable, but to conduct the debate one would first have to have a long discussion about what it means to be ''the best.'' If it means the most serious or the most substantive, then Copland hardly measures up - not in 1970 or in any other year - to Elliott Carter, Milton Babbitt or Roger Sessions. If it means the most innovative, then there is practically no comparison between Copland and, say, John Cage, Morton Feldman or Harry Partch. If it means the most financially successful, then Copland is easily outclassed by Gian Carlo Menotti and Philip Glass. But if it means the most famous - in the sense of the long, international run - then Copland indeed is a prime candidate for the honor. And if it also means the most respected, then Copland is the winner hands down.

Much to the dismay of its champions, American music as a whole is still not much known beyond these borders. Aside from whatever rock musicians are currently fashionable, the average European or Asian would be hard-pressed to come up with more than a few names of American composers. The list would be dominated, I'm sure, by artists associated with various forms of popular music: Duke Ellington, John Philip Sousa, Glenn Miller, Stephen Foster, Scott Joplin, Thelonious Monk, George Gershwin. Charles Ives might be included; in the field of classical music, the only sure bet would be Copland.

The reasons for Copland's enormous fame and respect, worldwide, are spelled out clearly enough in ''Copland: Since 1943'' (St. Martin's Press; $29.95), the recently published second half of the ''oral history'' autobiography the composer produced in conjunction with Vivian Perlis.

Copland's contributions to the 463-page volume are predictably modest; he has never been someone to toot his own horn, and the first-person accounts attributed to him focus primarily on the simple facts of where he went, who he met, how and why he happened to write certain of his better-known pieces. Nor is there much gushing in Perlis' lucid ''interlude'' commentaries, or in her 50 or so supplementary interviews with composers, performers and others who over the years have known Copland on more or less intimate terms.

But the Copland story speaks for itself, and the various assessments of it only amplify its message. Copland was the one who blazed the trail, in the 1930s and '40s, for American composers who sought a nationalistic musical identity, and during his nearly two decades as chairman of the composition department at the Tanglewood summer school he helped steer the careers of countless aspirants. In 1950 he won an Academy Award for his score for the film ''The Heiress''; a few years later, and not just because another of his film scores was titled ''The Red Pony,'' he was the foremost of American composers hauled in by Senator Joseph McCarthy and publicly accused of unpatriotic political activity. In the 1960s he was the White House guest of both the Kennedy and the Johnson administrations; he was the recipient of the Medal of Freedom and numerous honorary degrees. In the 1970s, after he had for the most part quit composing, he continued to travel the globe as a conductor and lecturer. In 1986 he became the first composer ever to have his music, and his ''vital contribution to American artistic life,'' celebrated by a Congressional Gold Medal.

The vital contribution has been as much personal as musical.

Copland's direct effect on other composers' scores was strongest in the World War II years. Although in his younger, less famous days he was considered an enfant terrible, he deliberately took a conservative turn in his search for a characteristically ''American'' music that might rally the spirit of his listeners. After the war, when dry Modernism became all the rage, he preferred to remain in the conservative vein, and eventually his younger colleagues dismissed him as old-fashioned. Later, with such orchestral works as the 1962 ''Connotations'' and the 1967 ''Inscape,'' he took a stab at serial and other more up-to-date techniques; the music came out as well-crafted, but sorely lacking in the warmth and lyricism that most listeners - including those of the younger generation - thought of as the ''Copland touch.''

But if the influence of his music waned, the influence of his personality did not. And he did not function, in the 1970s and '80s, merely as a celebratory.

As Perlis puts it in her concluding chapter: ''Those who have become acquainted with him only recently might be reminded that within the familiar genial, avuncular figure is a fighter for American music, a strong leader, one who has done more for composers and for American music than any other one person in this century. By now, it is rare to find anyone connected to 20th-century music who has not had a Copland experience. . . .

''It was a Copland talent to face each occasion with anticipation, fresh spirit and genuine pleasure. He has shared himself with so many musicians and listeners that he has become part of the American consciousness. The media helped to make familiar the tall, lanky figure with the spectacles and quick grin, but the enduring affection of the public for Copland has more to do with the vast accumulation of his sharing of himself, the continuous sound of his music in the collective ear and the successful transference of his own pleasure in making music.''

Copland is set to turn 90 on Nov. 14 of this year, but he is reportedly not at all in good health. Nine decades of sharing himself, it seems, has finally taken its toll, and it is very likely that his last public appearance was indeed his last.

His position is unique. Plenty of other American composers are just as skilled as he has been. Many are more prolific; many, arguably, are more profound, or more adventurous, or more innovative. And many have shown equal dedication as teachers and lobbyists. But none has produced music that resonates so strongly in this country's ''collective ear''; none has created a body of music so identifiably ''American'' as Copland did with that handful of works from the '30s and '40s.