Music sets the tone for girls' fashion trends.

While actual teen-agers are already twentysomething wannabes, it's the "tweens" — girls who aren't quite teens yet — who may style themselves after Britney Spears, Christina Aguilera and Mandy Moore, songstresses-turned-sirens with low-slung hiphuggers and halters.

The latest hopeful to target the tween fashion market: Josie and the Pussycats, the new live-action movie about those old cartoon rock 'n' rollers.

The movie's costumes reflect what's hot in the stores, and it's accompanied by its own line of Josie-branded clothes, from shoes to swimsuits.

"Girls of this age ... what rules their live is music," says Annemarie Iverson, editor in chief of YM magazine. "Music is of the moment, especially now when there are a lot of young stars."

The Pussycats — played by Rachael Leigh Cook, Rosario Dawson and Tara Reid — wear tight second skins of dark denim, beaded tank tops and high heels, a racy wardrobe for a bunch of high schoolers from Riverdale, USA.

But the costume designer, Leesa Evans, says, "This is not so much about trying to be sexy. It was about what they felt good in."

Evans, now at work on the live-action Scooby-Doo in Australia, says she thought hard about what girls would come away with from seeing Josie.

"I felt a huge responsibility. The subtle message is that the girls (in the movie) had fun with the clothes but they didn't have to follow the hottest trends."

She hopes they will set them. While headbands adorned with animal-print ears might not land on every tween's head, asymmetrical tops and low-rider jeans with extreme oversized legs are likely to catch on, Evans predicts.

Halters, flower prints, denim, preppy styles of the early 1980s and hiphuggers are all trendy with the tween crowd, reports YM's Iverson.

And nothing makes clothes trendy faster than the seal of approval from kids with baby-sitting money in their pockets.

"These girls are instantly reflective of trends. I don't know if they start them or if they take them from music videos or what, but trends trickle up and trickle down from them very quickly," says Iverson.

Designer Betsey Johnson recognizes the market power of tween girls — whether they're spending their own money or their parents'.

"Five-year-olds want to look like their 12-year-old sisters," says Johnson. "Teens and young twentysomethings look great, and little girls want to do the same — and do it faster."

Johnson makes matching outfits for 8- to 14-year-olds and their moms. For the pint-size versions, the necklines are raised and the skirts are lowered.

"It's age appropriate with an edge," says Kim Hingley, Betsey Johnson's vice president of sales.

Age-appropriateness isn't easy to define. Tweens often are caught between youthful experimentation and looks that parents may find too sexy.

"You can't dictate an exact age when to allow your daughter to wear what," says Iverson. "As soon as girls have an interest, there should be a respectful conversation (with parents) about clothes and the signs they might give off."