"If I were speaking to someone who was 90, I couldn't brag about being 80," is the characteristically philosophical way John Cage summarized his thoughts on his impending 80th birthday. Speaking by telephone from Paris the other day, he added quietly, "I think of each year as a gift." 

Actually, the prolific avant-gardist will not officially become an octogenarian until Sept. 5. But the music world is in the habit of throwing extended parties for its more colorful and influential composers, and the celebrations of Mr. Cage's next birthday have been under way around the world since January. On his current visit to Europe, he heard performances of his music in Italy and Czechoslovakia, and he plans to attend a festival of his music in Frankfurt in August. 

But Mr. Cage said he would be back in New York, where he lives, in time for another installment of his party, the Summergarden concerts that begin tomorrow night in the Abby Aldrich Rockefeller Sculpture Garden of the Museum of Modern Art. The 10-week series of free Friday and Saturday evening concerts is devoted mostly to music Mr. Cage has written since 1985. The performers are students from the Juilliard School, directed by Paul Zukofsky.

Mr. Cage is a soft-spoken, mercurial figure who started a revolution by presenting the proposition that composers could jettison the musical language that had evolved from medieval times through the mid-20th century, thus opening the door to Minimalism, performance art and various other branches of avant-gardism. Traditionalists regard him as an anarchist who led contemporary music astray. But for many new-music fans, over several generations, he is a liberator. 

Mr. Cage's music of the last several decades relies on a combination of chance elements and a kind of game playing in which performers are given general (if sometimes complex) rules that they can apply at their own discretion. 

This week's programs include "44 ," a 72-minute percussion quartet Mr. Cage composed in 1991, using notation that Mr. Cage calls "time brackets." These are sections lasting 75 seconds, during which the musicians must begin playing during the first 45 seconds, and must end the passage between the 30th and 75th second. Decisions about when to start and stop and about what tempos and instruments to use are left to the players. 

Mr. Cage's writings are sometimes governed by similar rules. His book "Empty Words" (1973-78), from which he is to read at next weekend's concerts, was written using chance operations. With the 14 volumes of Henry David Thoreau's journals as his source material, Mr. Cage composed his text one letter at a time. 

"The process," he said, "would tell me what volume, page and line to use, and then I would count the letters to find out which letter I should take. There is no meaning in the text, only sounds. It is a use of the voice that resembles music more than poetry." Welcomes Street Noise 

Mr. Cage also depends on chance elements that are beyond the scope of his rules and his performers' whims. There is the intrusion of noise, for instance. Although critics have complained that the museum's sculpture garden -- outdoors in the heart of Manhattan -- is hardly an ideal place to listen to music, Mr. Cage is counting on traffic noise to fill out the silent stretches in some of his works. 

"A work like "44 " includes a great deal of silence," he said, "and I think that will be good in Summergarden, because there is so much sound there already that there will always be something to hear. The noise is not so much a part of the piece as part of the experience."
Mr. Cage's free-spirited approach to composition, and to performers' leeway, raises questions about how his works should be regarded. To what extent, for instance, does he expect two performances of the same work to differ?
"In every degree," he said. 

In that case, if he were to walk in on one of his works, would he know which one it was, or even if it was his? 

"I have had the experience of not recognizing my own work," he admitted. "On the other hand, I have also had the experience of recognizing it, even though the instruments, durations and other elements were different." 

Has Mr. Cage ever, just for a change, felt like returning to his earlier style of precisely notated works, with set meters and recurring themes? Invention the Mother of Necessity
"Not really," he said. "I did it once recently, in a piece for organ called 'Souvenir.' When the organist who commissioned it told me that was what he wanted, I sent the check back. But he insisted, so I did it. 

"But I try to do what I feel is necessary to do. And my necessity comes from my sense of invention." 

He was reminded of a quotation about him attributed to Arnold Schoenberg, the 12-tone composer with whom Mr. Cage studied for two years in the mid-1930's. Schoenberg described his student as "not a composer but an inventor -- of genius." Did that bother Mr. Cage? 

"Oh no," he said. "Schoenberg was a marvelous person. I was happy to have him say anything. He gave his students little comfort. When we followed the rules in writing counterpoint, he would say, 'Why don't you take a little liberty?' And when we took liberties, he would say, 'Don't you know the rules?' You know, right now I'm reading his 'Harmonielehre,' and I'm really enjoying it, because at the end he says very clearly that there will one day be harmony for which there can be no theory." 

"The thing that always annoyed me about harmony," Mr. Cage added, "was that it was governed by laws. But you know, those laws need not be taken seriously."
Photo: "I think of each year as a gift," said John Cage, nearing 80. (Gene Bagnato)