Unlike most of her "The O.C." co-stars, not to mention the 1990 cast of blueprint series "Beverly Hills, 90210," Mischa Barton is actually still in high school.

If this comes as a surprise, you're not alone.

"People always guess I'm like 23, and I'm like, 'No, I'm really only 17,' " Barton, who will graduate from the Professional Children's School in New York this spring, mused recently. "Then people sort of insist I have to be older, and I'm like, 'No, I really am only 17.' I think it's funny. I have a 15-year-old sister who I swear to God looks like she's 30. So I mean, who knows? We grow up fast these days, I guess."

On the set of "The O.C.," 25-year-old Benjamin McKenzie (who plays love interest Ryan Atwood) and 23-year-old Adam Brody (neighbor Seth Cohen) act as big brothers to Barton. "We watch out for her," Brody said. Not that she needs it.

"She's not naive," Brody admitted. "She's more poised than us."

"She has the most experience of us all," McKenzie noted. "It's my first job and it's her hundredth, or whatever. She's really sweet and a total pro, very easy to work with."

"The O.C." is not quite the London-born Barton's 100th gig, but she has been acting since she was cast in the off-Broadway play "Slavs" at age 9. She continued in theater, appearing in Lincoln Center and New York Shakespeare Festival production, while co-starring in three seasons of "All My Children."

Barton has since appeared in more than a dozen movies both large (remember the ghost with the secret in "The Sixth Sense"?) and small (1997's "Lawn Dogs" with Sam Rockwell, 2000's "Skipped Parts" with Drew Barrymore). She also had a memorable role as a teenage lesbian in the final season of "Once and Again."

In recent months, Barton's appeared in Enrique Iglesias' steamy "Addicted" video, in commercials for Neutrogena and as the latest "It" girl in countless magazines.

"It's very flattering, I guess, to be considered so popular at a young age," Barton said. "It's kinda cool."

When it comes to discussing the breakout FOX drama "The O.C.," Barton shows the maturity her co-stars described. Asked if she expected its success, she replied, "No, definitely not.

"I don't think that's something that can be expected," she explained. "I mean, you get very distanced from it and you do your work and you think, 'I hope people think this is as good as I and our cast members feel it is.' Nobody was sure if anybody would care about a show about Orange County or if it would catch on, because it's kind of an edgy take and a new kind of conglomeration of a lot of things in pop culture."

Barton credits her co-stars for her humble outlook.

"The great thing is, none of us was intimidating to each other, nobody has an attitude," she said. In fact, "We definitely have way too much fun. Like, everybody says working on a show can be grueling and maybe not the funnest thing in the world, or that work gets them down. And I'm like, 'No way.' We have so much fun on our show."

"We're all pretty good friends," Brody added. "We all hang out a lot off the set. It's good. I actually feel like I made a cool little family on this show."

Next up for Barton is an independent movie in which she joins a cult. "Octane," due later this year, co-stars Madeline Stowe ("We Were Soldiers") and Jonathan Rhys-Meyers ("Bend It Like Beckham").

"It was a really cool cast and we filmed it in Europe. It was an action thriller, which was really fun," said Barton, who is reading scripts for her next film. "I'm just looking for a good movie project to do in my spare time, that I can get really excited about."

Barton is a big music fan who prefers the White Stripes, punk and hip-hop over Britney Spears. But unlike the only high-school student in the "Beverly Hills, 90210" cast (Brian Austin Green), she's not recording an album. Yet.