TEENAGE sex is a subject guaranteed to produce anxiety in adults. As teen pregnancy rates rise and the age of sexual initiation drops, the hand-wringing - and sense of parental helplessness - increases. But seldom has the public received a closer, darker look at adolescent sexuality than in the past month, thanks to a disturbing film and a controversial ad campaign. Early in the movie ''Kids,'' Larry Clark's bleak portrayal of 1990s adolescence, a predatory boy named Telly boasts to a friend about his latest sexual conquest - a barely pubescent girl. Explaining his fondness for ever-younger partners, he says bluntly, ''I like 'em new.'' That attitude also runs as a subtle theme through ads for Calvin Klein jeans. Those ads, showing young teens with come-hither stares and jeans pushed provocatively below their hips, sparked a public outcry. Critics - parents, conservative politicians, and child welfare groups - called the poses suggestive and exploitative, bordering on child pornography. For the first time in 15 years of producing ever more erotic ads, Mr. Klein withdrew his TV commercials, billboards, and magazine ads. Yet his public stance remains defensive. In a full-page newspaper ad last week, he disingenuously claimed that young people ''have very strongly defined lines of what they will and will not do - and have a great ability to know who they are and who they want to be.'' That flattering appraisal aptly describes some teens. But tell it to the baby-faced girls being persuaded to lose their virginity in ''Kids.'' Or tell it to any teenager who has ever felt weak-willed in the midst of peer pressure that demands: Go along with the crowd or lose face - and friends. Teenagers have always created their own culture, deliberately excluding adults. Yet they are also products of the larger culture, which they don't create. Teenagers are not the ones putting sex and violence on TV and in movies. They are not the ones creating the ad campaigns that test the limits of acceptability. And they are not the ones with the power to fill airwaves with vulgarity of the type that forced disk jockey Howard Stern to pay $1.7 million to the government last week to settle accusations of indecency in his radio broadcasts. Again and again, advertising and entertainment images that would have been considered beyond the pale 10 years ago now merit only a yawn. As Gen. Colin Powell observed in a speech in San Diego last month, ''Nothing seems to shame or outrage us anymore.'' Who needs a subscription to Playboy, for instance, now that local newspapers run full-page ads for lingerie featuring models in various states of near-total undress? The latest underwear ads have even gone coed. Calvin Klein himself currently uses topless teenage models in coed ads for his fragrance, ''CK one.'' Who can expect teenagers to be immune to the persuasive and pervasive sexual undertones of such advertising, whether the product is underwear, fragrance, cigarettes, or liquor? In his printed non-apology, Calvin Klein sanctimoniously acknowledged that his company has ''a special responsibility to young people - in fact, we share the concerns some have raised about the challenges children face growing up today.'' It will take more than his retreat to change popular culture and demonstrate real concern for the challenges children face. Calvin Klein and other advertisers will be back with new approaches. But perhaps the successful protests against his ads mark a small turning point in reconsidering the boundaries of public taste. The issue isn't a question of censorship but of understanding the enormous power of media images. Some critics view ''Kids'' as a needed ''wake-up call'' for parents about the perils of adolescence - sex, drugs, alcohol, casual violence. Perhaps the Calvin Klein ads belong in the same wake-up category. Glossily seductive advertising hardly comes for free. The consumer pays the cost - and in ways that often go far beyond mere dollars and cents.