WHEN ROBERT Joffrey founded the ballet troupe that bears his name, he was 25 years old. This W season, the Joffrey Ballet itself has attained that age, and a distinctly celebratory air will surround the company's coming visit to the Kennedy Center Opera House, starting next Saturday evening and running through March 14. The anniversary spurs reflections on the unique niche the Joffrey company has carved for itself within the spectrum of contemporary ballet. Perhaps the most distinctive thing about the Joffrey is that, among the nation's major troupes, it is the one most clearly and completely American in character, aim and atmosphere. The oft-repeated story of its humble beginnings seems as much a part of American folklore as of ballet history.

In 1956, Joffrey dispatched a company of six dancers (Glen Tetley among them) on a whistle-stop tour in a borrowed station wagon, while he stayed behind in New York to support the operation by teaching; all the balletsthe group performed were concocted by Joffrey himself.

Today Joffrey is a pillar of the international ballet community. The company has grown to 39 dancers, has an annual budget of $5.4 million, and is the country's third largest ballet institution. Yet the company has kept faith with its origins as a rambunctious Yankee upstart--it retains its sense of youthful verve and bravado; it continues to seek a non-specialized, broad-based public; and it has amassed a uniquely diversified repertory combining voguishness, tradition and a zest for the unconventional, a mixture neatly reflected in the approaching Kennedy Center programs. It was no accident that when the long-running Dance in America series made its debut on public television in 1976, it was the Joffrey Ballet that was chosen to take the initial plunge.

It is certainly the case that American Ballet Theatre and the New York City Ballet, the biggest of the nation's classical troupes, are American enterprises in very real ways. ABT was nurtured through most of its existence by native-born Lucia Chase, and has championed a horde of indigenous choreographers, Agnes de Mille and Jerome Robbins among them. Russian-born George Balanchine and American Lincoln Kirstein, founders of the NYCB, called their first company the American Ballet, have always adored and cultivated American dancers, and have fostered a decidedly New World approach to classicism. But ABT has long depended heavily on mostly foreignsuperstars for its appeal (it's now headed by one of them, Mikhail Baryshnikov) and imported, evening-length 19th-century warhorses have formed a prime buttress for its repertory. Balanchine, inevitably, has drawn repeatedly on his Russian forebears and background for inspiration. And both ABT and NYCB are opera house companies, by virtue of their size, opulence and repertory.

By contrast, the Joffrey Ballet, though it has played and does play in opera houses, is far more geared towards informality, and an audience that may never have heard of Marius Petipa, the archetypal choreographer of the Russian imperium. With its accent on democratic ensemble and sharing of roles, the Joffrey company has never developed stars, though it has had many very fine dancers.

It has never presented any of the traditional, full-length 19th-century classics, not even--mirabile dictu--"Nutcracker." It was the Joffrey, moreover, that introduced rock music to the ballet stage, in ballets such as Joffrey's own psychedelic "Astarte" and Arpino's "Trinity," and it has been Joffrey who has taken the lead in bringing modern dance choreographers within the ballet orbit, with recent works by Tharp, Dean, and Moses Pendleton, and in earlier days, with works by Tetley, Anna Sokolow, Jose Limon, Norman Walker, Louis Johnson, and Remy Charlip.

This is not to say the Joffrey Ballet has no connections with ballet tradition. But the links, except for the historical principles of academic training that underlie all ballet dancing, are to a very large extent with the traditions of the present century. Indeed, one of the foremost accomplishments of the company has been a spate of reconstructions and revivals of significant 20th-century choreography, ranging from treasures of the Diaghilev era (Massine's "Parade," Fokine's "Petrushka," and Nijinsky's "Afternoon of a Faun," for example), to the mid-European expressionism of Kurt Jooss ("The Green Table" and other ballets) to past work by Frederick Ashton, Cranko, de Mille, Robbins and even Balanchine. These projects have been undertaken, for the most part, with exemplary scholarship and devotion, and wherever possible, with the assistance of living choreographers and dancers involved in the original productions. At the same time, Joffrey has admirably succeeded in avoiding the deadly museum-piece syndrome--the company has found its own ways of infusing such material with new, and genuine, life.

Highlighting the upcoming Kennedy Center programs will be the year's new repertory acquisitions--"Light Rain," by the company's associate director and resident choreographer Gerald Arpino; the first American staging of John Cranko's full-length "The Taming of the Shrew"; and Jiri Kylian's "Transfigured Night," to the historic Schoenberg score of that title. A special Anniversary Gala on March 10 will feature ballets by Joffrey, Arpino, Kurt Jooss and Arthur Saint-Leon, and other evenings will include works by Arpino, Kylian, Twyla Tharp, Laura Dean, Ruthanna Boris and Marjorie Mussman--a typically wide-angle Joffrey assortment.

The very breadth and versatility of the Joffrey troupe has had its dangers--in trying to do such a multiplicity of genres, styles and idioms well, one runs the risk of doing none of them to perfection. For the dancers, moreover, the lack of single-minded commitment to a particular style can sometimes induce a bland anonymity in performance--everything, from Massine to Tharp, can begin to look too much alike. Yet these are perils Joffrey has been willing to hazard, for the sake of an artistic attitude of characteristically American generosity, one that is hospitable to all shades of creative expression. There's no doubt that American ballet would have been much the poorer without it.