Fifty years ago it took a talented adult novelist to capture the wayward voice of a generation in "The Catcher in the Rye." These days, kids just do it themselves. Or at least one kid has. Nick McDonell was 17 when he wrote "Twelve" (Grove Press), a nihilistic novel appearing in bookstores next week. McDonell is no J. D. Salinger. But he's smart enough to know that. Ask him if he doesn't think the characters in his book aren't a little, ah, thinly drawn, and he quickly agrees: "My characters are caricatures of themselves." He could've argued that the teens he writes about are disaffected prep-school louts from the Upper East Side of Manhattan with too few brains, too much money and way too much access to all sorts of drugs. In other words, they're supposed to be so shallow that they're hard to tell apart. Instead he does the stand-up thing and tells the truth: "I'm not good enough yet to do full-blown psychological portraits." What he does know how to do is establish a mood (completely creepy) that he sustains to the bitter, blood-soaked end. And he knows how to make you keep turning pages--his book generates a nasty narrative undertow that writers twice McDonell's age would pay dearly to pull off. He makes it look so easy that people talk less about his talent and more about the fact that his father is Terry McDonell, editor of SportsIllustrated and good friend of Morgan Entrekin, publisher of Grove Press. Which raises the reasonable question, would any of this have happened had Nick McDonell not been born into the right family with the right connections? Probably not. Is his novel nevertheless an arresting debut? Absolutely. Nepotism works best when it doesn't need to.