In a recent book, Harvard psychologist Dan Kindlon claims languishing Ophelias are a thing of the past: Brainy, assertive “alpha girls” are taking over American high schools. While his theory’s open to debate, its Exhibit A could be Juno MacGuff (Ellen Page), the 16-year-old heroine of this quirky hit comedy, who already seems like a potential icon for Generation Y. Not since Fargo has a pregnant woman mouthed off this much on film — or kicked as much ass, metaphorically speaking.

Until now, teen pregnancy was the province of cautionary after-school specials and movies on the Lifetime Network. The typical heroine was a Good Girl (cheerleader or nerd) who made a Mistake with her boyfriend or let the loutish quarterback Take Advantage. While she wept tears of lost innocence, various adults — parents, counselors — fretted over her situation. Juno, by contrast, is no sheltered flower: She refers casually to abusing booze and Adderall, and the film strongly implies that getting knocked up by her friend Paulie (Michael Cera) wasn’t her first sexual experience, though it was his. She was the one who took his virtue, and she’s also the one who deals with the consequences. That’s probably a good thing, since meek, sweet Paulie looks about as ready to be a father as a newly hatched chick.

Juno does follow in the footsteps of many a cinematic pregnant teen when she decides to carry the baby to term. (The movie carefully frames this as a personal choice.) But instead of asking adults for help, she magisterially announces to her dad and step-mom that she’s found her baby an adoptive family. Then she inserts herself into the lives of the barren, thirtysomething couple of her choice (Jennifer Garner and Jason Bateman) to make sure they’re fit to raise her offspring.

All of this could have been the recipe for an insufferable movie about a precocious teen — Ferris Bueller on prenatal vitamins. But Juno has two huge things going for it: the inventive script by Diablo Cody and the heartfelt performances. Like Heathers in 1989, Juno has a language that’s all its own — it’s safe to say that no quick-stop clerk would ever say, “This is one doodle that can’t be undid, home skillet,” any more than a high school girl would ask, “Veronica, are you pulling my dick?” But, wacky as it is, Juno’s wordplay feels rooted in the world of Internet message boards, where people have time to compose their snappy rejoinders. (“Honest to blog?” Juno’s friend asks when she hears about the pregnancy.) It’s not hard to imagine some of the lines making it into the realm of real slang, similar to Heathers’ “What’s your damage?”

After scaring audiences — particularly male ones — with her role as a young girl who turns the tables on a pedophile in Hard Candy, 20-year-old Page is just as eerily self-possessed here, but more pleasant to watch. When she shows us Juno’s vulnerability, it doesn’t feel like a radical shift of gears, just a new facet of a person we’re getting to know. In a comedy as stylized as this one, it’s easy for supporting characters to slip into stereotypes — the Clueless Parents, the Shallow Yuppies. But director Jason Reitman and his cast generally avoid those broad strokes. As Juno’s parental units, TV character actors Allison Janney and J.K. Simmons tolerate her bossing with seen-it-all Midwestern aplomb. Garner makes us believe in her aching desire for a child, and Bateman is all too recognizable as the Gen X guy who isn’t ready to stop being a kid himself.

The movie has its flaws. The incessant indie-folk-pop soundtrack gets twee very fast, and it seems a little soft for Juno, a self-declared devotee of Patti Smith. A certain development toward the film’s end feels forced, as if somebody decided it needed a shot of conventional romanticism. Juno doesn’t resolve the question of whether the Alpha Girl will ever find a mate who’s her equal. But it transcends its own formidable cuteness factor to become a movie with staying power.