Two-and-a-half decades have gone since Yves Matthieu Saint Laurent was first hailed as the boy wonder of fashion. In that time the reticent designer has become its foremost world influence. Today he rules over a global empire of clothes, perfumes, and accessories which, by some estimates, is valued at close to $2 billion retail a year.
Now the Paris couturier is being apotheosized at the Metropolitan Museum of Art here with a retrospective exhibition of his works. He is the first living designer to be so honored by the museum's Costume Institute. ''Yves Saint Laurent: 25 Years of Design,'' conceived by Diana Vreeland, special consultant to the costume wing, has just opened - with great fanfare.
''Why Saint Laurent?'' Mrs. Vreeland writes in the show's catalog. ''Because he is a genius. . . . He is followed across the oceans of the world by women who look young, live young, and are young, no matter what their age.'' The drawing power of Mr. Saint Laurent, in combination with the Met and the indomitable Mrs. Vreeland, a former editor in chief of Vogue, has so far been magical. The media responded to the press preview with rivers of ink and miles of film. Guests came to the private benefit gala in unprecedented numbers: 810 for dinner at $500 a ticket and 2,500 later to dance at $100 an entry.
A measure of the Saint Laurent show's big appeal lies in the shock of recognition. ''We can all relate to it,'' says Jean Druesedow, assistant curator of the Costume Institute. ''We have an instant sense of identification with the exhibits. Almost all of us have worn copies of the clothes, if nothing else.''
There is indeed a pleasurable feeling when we see Mrs. Vreeland's lineup of pantsuits and recall how Saint Laurent's deft handling of men's tailoring for women encouraged them to wear trousers with chic, comfortable assurance as early as the late 1960s.
Other groupings in the exhibition of about 240 pieces have familiar associations, too. Saint Laurent himself took note of the fashion themes that have recurred throughout his career as he strolled through the exhibition, signing autographs and posing for photographers at the press opening. ''I can see my style through the years,'' he said. ''It makes me very emotional.''
There are, for example, the safari looks he began for street wear in 1958, when he was the fledgling 22-year-old designer for Christian Dior. Such nautical ideas as the sailor's pea jacket (his first one dates to 1962), his famous feminine version of the male's tuxedo, and the stunning Russian-inspired fashions that set off a rage for full-skirted ''rich peasant'' dressing in 1976 all strike chords in the memory. Many of these styles have lasted and are still with us in one form or another.
Saint Laurent's strong affinity for the arts, which goes back to his childhood, takes up a large portion of the show. Brilliant examples from the Picasso-influenced collection of 1979, sumptuous gowns with Shakespearean, Velazquez, and Winterhalter overtones, as well as his much-imitated 1965 Mondrian color-block suits and dresses, are effectively displayed. His witty 1966 Pop-Art clothes are hung like paintings on the walls of the Costume Institute's galleries.
Also included are selections from wardrobes YSL has made for Catherine Deneuve to wear in various films and for French singer-dancer Zizi Jeanmaire to wear on stage. So is the uniform he designed for writer Marguerite Yourcenar's induction into the Academie Francaise.
The exhibition was funded by a $350,000 contribution from Gustav Zumsteg, head of Abraham, the Zurich silk firm. His backing has not raised questions of ''commercial taint.'' (''You can pretend you're an ivory tower, but few museums today can afford to put on special shows without outside financial help,'' says Jean Druesedow.)
The monetary edge such an exhibition gives a living designer whose 208 licensed products alone bring in $700 million annually is a gray area, however. But Saint Laurent is no stranger to controversy. Known to have a moody, introspective nature, he also has a reputation for breaking conventions and scandalizing his public. Visitors to the show will be reminded of the to-do he caused with his see-through clothes. And few can forget the ruckus provoked by the bacchanale launching of his ''Opium'' fragrance. His 1959 beatnik collection of black leathers and turtlenecks so horrified the conservative clientele of Dior that it cost him his job there.
As things turned out, the clothes were prophetic, and Saint Laurent went on to establish his own house, win international acclaim, and enjoy the triumph of his current show at the Met.