"Sixteen and a half," she pointed out during an interview in her dressing room on the set of "What I Like About You," a new sitcom on WB. Still stoked from a marathon PSAT session with her studio-appointed tutor ("There are these things called mnemonic devices," she explained, "which are sentences that help you remember words like `ameliorate' "), Ms. Bynes reflected on a distinguished career that began way back in the Clinton administration.

After being discovered at a children's comedy camp in Los Angeles in 1994, Ms. Bynes quickly emerged as the brightest talent on the Nickelodeon network who wasn't an animated chihuahua. First as a sketch-comedy performer on "All That," a sort of pint-size "Saturday Night Live," and later as host of "The Amanda Show," on which she parodied pop icons as lofty as Judge Judy, Ms. Bynes developed a following among the post-Barney set that was like a stealth explosion.

"A couple years ago, any kid 6 to 13 knew right away who she was," said Dan Schneider, who has produced nearly everything Ms. Bynes has done as an actress. "But most parents were like, `Amanda who?' "

That's slowly starting to change. Ms. Bynes is a product of an unofficial new pipeline program in which young performers start on Nickelodeon's low-budget, quick-turnaround shows and then, if they succeed, break out into sitcoms and movies.

Like Ms. Bynes, Kenan Thompson and Kel Mitchell walked away from "All That" with their own Nickelodeon series, "Kenan & Kel," before branching out into films like "The Good Burger" (1997). Nick Cannon has followed his "All That" run with "The Nick Cannon Show," a new Nickelodeon musical comedy program, as well as the lead in "Drumline," a big-screen comedy that opens in January. Waiting in the wings is Jamie Lynn Spears, Britney's 12-year-old sister, a sparkly-eyed standout in the current cast of "All That."

"We hate to see our kids leave," said Kevin Kay, Nickelodeon's vice president for development, "but fortunately, we have something called reruns, and the more our stars do on the outside, the better it reflects on us."

With a gift for physical comedy and a fearlessness in front of live studio audiences, Ms. Bynes helped make "All That" and "The Amanda Show" the top-rated live-action programs on Nickelodeon. (Proving Mr. Kay's point, reruns of "The Amanda Show" on Saturday nights still outperform the channel's other non-animated shows, even though it's been two years since an original episode was made.) Young girls admired her because she's pretty and they wanted to be like her, while the boys appreciated that Ms. Bynes wasn't afraid to stick her head in a bucket of mud or fall off a roof.

Mr. Schneider, who co-created those shows, then wrote Ms. Bynes into a 2002 feature film, "Big Fat Liar," and later cast her in "What I Like About You," a program he created with Wil Cahoun. In it, Ms. Bynes plays a suburban teenager who moves in with her reluctant big sister, played by Jennie Garth, in Manhattan after their father takes a job in Japan.

WB may claim Ms. Bynes now (the network's Web site lets fans download a life-size image of her in 28 8 1/2-by-11 printouts), but she literally grew up on Nickelodeon. She sprouted six inches during her years there. "We went through seven or eight stunt doubles," she said. "They couldn't keep up." But the material was also getting old. "I knew I didn't want to be a Nickelodeon kid when I was 30," she said. "I was having fun but at 15, you don't want to be doing what you did when you were 12." What she likes about "What I Like About You," she said, is "it has appeal to people twice my age, even older." Since the sitcom debuted in September, it has been the top-rated show in its Friday-night time slot among women in the desirable 18-to-34 age group.

Many a "True Hollywood Story" has been told about show-business kids going bad, but Ms. Bynes seems remarkably self-possessed and far more sedate than the highly caffeinated characters she plays on television. Her idea of a big night usually revolves around a rented video and microwave popcorn, she said.

Mr. Schneider, a former child star himself, having appeared on the 1980's sitcom "Head of the Class," said: "I've seen kids in her position experiment with drugs and be too promiscuous, but Amanda has avoided all of that. My wife, who knows her, says she's almost like Marcia Brady in that she's so clean-cut and wholesome."

Still, balancing long days of filming with prep courses for standardized tests is enough to make anybody a bit world-weary. "Some days," Ms. Bynes said, half-joking, "I'd like nothing better than to sit on the couch eating bonbons. It's like, can I just have a blanket and be 5 again? Please!"