Paris — In spite of all passing fads and fantasies, jeans have become a veritable unisex uniform for 21-year-olds of all ages. Like those old soldiers, ''they never die; they just fade away'' - particularly when instantly aged by highly specialized washing.
Jeans have even attained museum status. The Musee des Arts Decoratifs, in one wing of the Louvre, displays one of the original pairs that Levi Strauss, the Bavarian immigrant who settled in San Francisco, cut out of that tough tent material for the gold prospectors back in the 1870s.
Actually the name ''denim'' originated in France. It is a contraction of the words for the sturdy cloth once known as serge de Nimes that was manufactured in the latter part of the 19th century in that old Roman town in Provence.
Although dozens of French firms turn out quantities of jeans - imitations and careful copies complete with top stitching, copper studs reinforcing the stress points, and the prime requisite of deep blue indigo denim - Levi Strauss, which holds the world's record for the largest turnover in clothing, leads the field in Europe.
Last May the company celebrated the 50 millionth pair of jeans manufactured in France alone. For the occasion, the renowned logo depicting two work horses attempting to tear apart a pair of Levi's was transformed by substituting a tiny Eiffel Tower in the center of the label.
Curiously enough, Levi's have become a kind of status symbol, along with those unmistakable Louis Vuitton accessories. Many Parisians collect the genuine jeans as judiciously as any antique dealer hoarding his treasures. Levi's are the substitute for designer jeans in Europe, as it seems doubtful that Yves Saint Laurent or Christian Dior would ever care to see their prestigious labels stitched across everyone's derriere a la Gloria Vanderbilt, Calvin Klein, etc.
Yet Claude Montana, the highly influential ready-to-wear designer, calls jeans ''the greatest invention of the 20th century.'' He proceeded to present prototypes in a luxurious wool and silk blend that sold fantastically well last summer at Biba in Paris and VIP in Saint Tropez.
With an eye on Montana's success, Levi Strauss has just signed up Jean-Charles de Castelbajac, another ready-to-wear star in the French firmament, to create a new line of jeans that will eventually hit the mass market next year.
Levi's famous ''501,'' authentic copies of the 19th-century originals, are deemed to have the classic appeal of an old car and account for 33 percent of the total turnover in denims. Twenty other styles range from hip-huggers cut so tight that if you have a coin in your back pocket you can tell whether it's ''heads or tails'' when you sit down, to various other models that do not necessitate a total respiratory stoppage in order to close the zipper.
Levi Strauss not only produces jeans in 40 countries, but also runs the specialized laundries where finished garments are processed to the requisite softness and standards of the slogan: ''The older they get, the better they look.''
The Levi laundry at Antwerp receives the entire production from Central Europe, shipped in from the various factories and warehouses scattered across the Continent, and currently handles 20,000 pairs of jeans daily. While this vast industrialized laundry, which opened last year, is highly automated, it still employs 140 workers alternating in two eight-hour shifts from 6 a.m. to 10 p.m. The balance of the European production is washed by subcontractors dealing mainly in stone-washed jeans, which account for 60 percent of the current turnover.
When the jeans arrive at Antwerp, the denim is stiff as cardboard. Four processes of the dark wash, including two different spins through the giant machines, the rinsing, and final chemical softener turn them out nearly as soft as feathers. A special vacuum machine flips the pants inside out for the washing procedure, after which they are tumble-dried and placed on another unique vacuum , which reverts the jeans to the right side before they are pressed, individually inspected, and packed for shipping.
Shrinkage is carefully calculated at a maximum of 3 percent, outmoding the old system of jumping in a rain barrel and allowing the garment to shape and dry on the wearer's body.
While the dark wash lends a certain patina, many fans demand instant aging achieved by stone washing - an entirely different treatment that will also be installed in the Antwerp laundry.
Time was when new clothes were supposed to look new, but ''distressed fabrics'' now appear to be totally a sign of the times - a trend that grizzled 19th-century prospectors inadvertently seemed to have launched as they battered and bashed their jeans against rocks while searching for those precious nuggets in the riverbeds. Today's stone washing is an updated version of that process - but it still uses stones.
With the Socialist regime in France, fashions appear to alternate between supreme luxury in the haute couture collections that only a very few can afford, and the opposite extreme of purposeful poverty, which is fast becoming a dire reality for many.
The faded, shabby jeans connote a certain countersnobbism. ''Today under the Socialist government, we even see blue jeans worn in the Elysees Palace,'' observes Jacques Mouclier, director of the Paris high-fashion syndicate.