SOME 10.5 million Americans - two-thirds of them men - schussed down mountains in the United States in 1993. About 3.7 million glided over snowy landscapes on cross-country skis. Another 1.8 million ``shredded'' the slopes on snowboards. Most snowboarders were teenage males, according to industry figures, many hardly older than the sport itself, which has been around about 15 years. Industry experts decline to speculate on the gender disparity in Alpine skiing, except to say that the difference is long-standing. Cross-country is almost gender neutral, and observers expect more women to pick up snowboarding as the sport gets better established. All three sports are growing. Most regions of the US are experiencing above-average snowfall, and sales of ski gear are up 8 percent this year.