Call it the revenge of the generation that did its homework to CHiPs and The Bionic Woman. From the snowboarding slopes of Colorado to the Port-A-San lines at Lollapalooza, X-slackers who grew up in the '70s and early '80s are donning baggy T-shirts emblazoned with the likes of Kojak, Richie Cunningham, and the Brady Bunch. Merchants have been quick to oblige this trend. Sessions, a rock duds catalog, hawks a shirt with Brady housekeeper Alice under the words ''Anyone for Flapjacks?'' Fuct, an L.A. company, offers skateboarding styles with morphed images from Planet of the Apes and Jaws. Meanwhile, Manhattan clothing store Na Na proudly stocks Charlie's Angels and Saturday Night Fever shirts. ''That was the stuff we wore when we were growing up,'' says Na Na assistant manager Doreen Younglove, 25. ''I idolized Donny and Marie, and Farrah Fawcett- Majors.'' And vintage tees that somehow escaped the trash can are particularly coveted. Ira Glick, the 25-year-old owner of Total Impact Shirts in Toronto, scored authentic novelty shirts from local head-shop owners. ''Battlestar Galactica, Star Wars, and Saturday Night Fever are huge sellers,'' he says. ''The shirts are a terrific icebreaker.'' Would Gary Coleman, whose gnomish face graces a Diff'rent Strokesshirt sold by Glick, don the tee? ''I'd wear it,'' proclaims Coleman, 26. ''If you're proud of yourself, you wear yourself.'' Guess that means that in 20 years, the Olsen twins will be sporting their tot likenesses with no sense of shame.