OK, you're hip enough to know that our cover girl, Miley Cyrus, has a rocker alter ego named "Hannah Montana" and elementary schoolers adore the 15-year-old. But did you know that this Queen of Tween World reigns over your world, too?

Of course, she's only a symbolic head of state, Montana or otherwise. Tween World's real rulers are her 8- to 12-year-old admirers: the kids be-"tween" infancy and teendom who affect us all, even if you've never driven soccer carpools or bought bling at Claire's Accessories.

Since Beatlemania unleashed Baby Boomers' power in the '60s, youth-driven cultural upheavals have erupted -- but those young people driving the trends were usually old enough to have a license. While protesting college students spurred the Vietnam War's end, their preteen siblings watched cartoons and awaited their day.

Not today's tweens. For the first time, kids who aren't old enough to be in middle school, let alone high school or college, are determining what cars, clothes, computers and music we buy, what movies and TV shows we watch, even how we talk and write.

"Our culture has become more youth-obsessed than ever," says Elayne Rapping, professor of American studies at New York's University at Buffalo. "Children are cool, and the older you get, the less cool you are. That's why women are trying to look younger and even dress like their children. Kids have more influence over their parents than vice versa."

J.R.R. Tolkien, author of the "Lord of the Rings" trilogy, is believed to have coined the term "tween" -- but for 20- to 32-year-olds. Only as kids, especially girls, were recognized for great purchasing power did advertisers define "tweens" as 'tween childhood and teenhood. "It has nothing to do with development, other than of early and loyal lifetime shoppers," says Robert Thompson, founding director of the Center for Television and Popular Culture at New York's Syracuse University. "If you can make an 8-year-old into a consumer, you potentially have her for 70 years."

Tweens spend or influence their parents to spend $500 billion a year, estimates children's marketing expert James U. McNeal -- enough to buy both Microsoft and Google. Advertisers, TV and movie producers, fashion designers and even car makers understand all too well that preteens' power extends far beyond bestowing superstar status on idols like Cyrus or Zac Efron. How? Read on.

The new entertainment

Turn on a TV and you may wonder, "Where have these young girls come from?" Try the Disney Channel and Nickelodeon. Their stars have erupted across the tube into shows and movies you probably wouldn't associate with the preteen set.

Courting such youth, the Country Music Television Awards chose Cyrus as host, while No. 2-rated "Dancing With the Stars" has showcased tween ballroom dancers, Disney teen band the Jonas Brothers and "High School Musical"'s Monique Coleman.

"You ignore kids at your peril," says "Dancing"'s executive producer Conrad Green. "They've got power over the remote control. Any adult says, 'I want to watch this' and appeals to reason. An 8-year-old girl says, 'I'm watching this, I'm watching this, I'm watching this!' and you give in."

At cinemas, cartoons or fantasy characters dominate. This summer's G-rated "Wall-E" hit $127 million by its second weekend, and last year's PG-rated "Shrek the Third" and "Alvin and the Chipmunks" earned a combined half a billion dollars, reports online movie publication "Box Office Mojo."

"PG has become the cool rating as filmmakers realize it can be a great way to bring in a big audience and earn a lot of money," says Paul Dergarabedian, president of box-office tracker Media by Numbers.

Fashion for big kids

When it comes to fashion, tween tastes are becoming impossible to ignore.

While tween idols Amanda Bynes and Hilary Duff have launched clothing lines for fans, other young stars are targeting an older market that could include you. Cyrus has said she wants to design Marc Jacobs-style duds for adults, and Lindsay Lohan has dipped her toe into selling footless tights. Stay tuned for more coverage.

Another fashion innovation with tween roots: customization. Call it the new monogram. But rather than stamping your initials on towels or wearing a boyfriend's letter jacket, you can get anything altered to your whims.

NikeID enables you to design your own shoe, Xbox 360 lets you create your own graffiti- and tattoo-adorned faceplate for the game console, and Apple lets you change out the case for your phone or laptop.

If you think top fashion designers are above it all, get real. In a nod to tween lust for "techno-bling" -- at any price -- Burberry, Gucci, Christian Dior and Chanel design iPod accessories.

"We're acting like kids to be cool," says pop culture expert Faith Popcorn.

The "My Decade"

Of course, for this generation of under-13s, doing anything online may be more common than brushing their teeth. MySpace, which started as a place for kids to strut their social networking selves, has been co-opted by adults, with 85% of users over age 18, according to a MySpace spokesperson. You now can find websites for MyCoke, My IBM and MySubaru. My Goodness!

"If the '70s were dubbed the Me Decade, this era could well be the My Decade," says culture reporter David Browne.

Driving car trends

Automakers have followed preteens to the Web -- because they're the ones who care about a car's look and features. "Tweens have $50 billion of influence on their parents' purchase of new and used vehicles, cars and pickups," says marketing expert McNeal. Scion sponsors Whyville.net, a popular tween website. "We want to reach the youngest crowd and gain their interest at the earliest age possible," says vice president Jack Hollis.

The new workplace

Now the bad news. Preteens may grow up to be your boss, and you may join professor Rapping in her frustration about being marginalized. "When I grew up, you were supposed to respect your elders," she says. "Now kids think they know everything -- and aren't about to listen."

Even worse, grown tweens might not be "down" with toiling. "In a push-button society, preteens are used to getting things immediately," says child psychologist Michael Osit, author of "Generation Text."

But the trade-off may be an invigorating youthful spirit. When did you laugh most with friends and think you could do anything? Your own tween years, no doubt. And remember, from infancy to the grave, we're all " 'tween" something.