The Human Truman Show (The Olsen twins were born 16 years ago. Now they're worth more than you.)

Mary-Kate and Ashley Olsen's real birthday, as many an 11-year-old girl can tell you, is June 13, 1986, but their first few months on earth don't really count--they didn't accomplish all that much. That changed in October 1986. Selected from a lineup for their nice smiles, the Olsen twins landed a shared part as newborn Michelle Tanner on ABC's Full House. In September 1987, one of them (your guess is as good as ours) was trotted out in front of the camera in the arms of actor John Stamos.

Since then, in their trajectory from the Full House twins to the Olsen twins to Mary-Kate and Ashley to the mary-kateandashley brand, the Olsens, who have just turned 16, have become the most financially successful child stars ever. After spending nearly their entire lives on television, they have become a real-life, human Truman Show. "They are a property now, aside from being people with a heartbeat," says Robert Thorne, 47, an entertainment lawyer who has managed the girls' careers since they were 4 and is now CEO at Dualstar Entertainment, which handles everything Olsen. (The "property" couldn't be interviewed for this story. It had midterms.)

To understand the business of Mary-Kate and Ashley, imagine Oprah Winfrey plus Martha Stewart plus Barbie. Dualstar, which was founded in 1993 to manage the girls' careers, generated $500 million in retail sales last year, mostly from its television and production businesses (Oprah). In 2002, Thorne expects Dualstar to generate almost $1 billion in revenue, boosted by a new fashion-and-accessories line at Wal-Mart (Martha). To date the twins have been television, video, and film stars; editors-in-chief of their own magazine; fashion designers; recording artists; executive producers; authors; videogame heroines; and Mattel models (Barbie).

If you don't have young kids, especially daughters, the Olsens' earning power might be hard to believe. Start with the direct-to-video business. Mary-Kate and Ashley have produced nearly 40 videos, sold more than 30 million copies, and generated $500 million in retail sales, according to Warner Home Video (owned by FORTUNE's parent). For the past two years they have dominated Billboard's Kid Video sales chart, and last year Our Lips Are Sealed was No. 11 in overall video sales--ahead of best-picture nominee Erin Brockovich (No. 14) and The Sopranos: The Complete First Season (No. 15).

The Olsens' first big-screen movie, It Takes Two, in which they starred alongside Steve Guttenberg, earned them $1.6 million, brought in $19 million in ticket sales, and then rallied $75 million in home-video sales, making it one of Warner's bestsellers in the family category. They have sold 29 million books despite being only in sophomore English class, and 1.5 million records even though they are, as one 11-year-old told us, "not very good at singing." They have the No. 2 girls' doll after Barbie and are on TV an average of 35 times per week. Now they sell capri pants. (This matters if you are 9.) In all, they are worth $38 million. Each. "We will out-earn any celebrity in America within two years," Thorne says. "Oprah. Martha. Julia Roberts. Anybody."

Those dollars will come from the wallets and pocketbooks of America's moms and dads. Thanks to their uncanny likeability (see We Hate 'Em! We Love 'Em!), the twins have a lock on the country's daughters. In the fame business, this quality is judged by the so-called Q score (i.e., the percentage of viewers who know a celebrity and have a favorable impression of him or her), and the Olsen's Q score is--and has been since they first emerged on camera--extraordinary. As preschoolers the Olsens had a higher Q rating for their age group than Michael J. Fox on Family Ties, and one rivaling Henry Winkler as the Fonz. "Currently, with the under-12 audience, they are in the top ten of all personalities," says Henry Schafer of Marketing Evaluations/TvQ, which computes the Q scores.

And this audience knows how to spend, or rather, how to get spent on. Tweens--the 27 million kids between the ages of 6 and 12, comprising 10% of the U.S. population--are avid consumers. (The first reference to "tweens," by the way, was in a 1938 Mickey Rooney movie, Love Finds Andy Hardy, when young Judy Garland sings, "I'm too old for toys and I'm too young for boys; I'm in-be-tween.") Last year, America spent $264 billion on the Olsen-addicted generation, according to Dr. James McNeal, president of McNeal & Kids Youth Marketing Consultants. Since the early-'90s, spending on tweens has grown at an annual rate of around 15%, McNeal says--much faster than any other demographic.

But building a business on two little girls is a tricky proposition. For one thing, they grow up. They make announcements like "they want to be movie stars," as Robert Thorne said recently, or that "they are retiring from television," the medium that made them famous. Then there are midterms, and boyfriends, and deciding what to wear to the Vanity Fair Oscars party. And waning Q scores: "It doesn't look like they are broadening their audience beyond kids," say Schafer. "Among teenagers they are very, very slightly above average. Smack-dab in the middle."

To be fair, critics were crying "too old" when the girls were 6. The same proof is almost always trotted out: Few child actors are able to carry their fame into adulthood. For every Jodie Foster, there are countless Barry "Greg Brady" Williamses waiting for their turn on Celebrity Boxing.

Thorne, though, argues that change is a secret of the Olsens' success. New shows, dolls, records, and videogames mimic the lives and concerns of the twins' fans, who are rapidly changing themselves. The girls grow with their audience. Plus, he's there to steer them. "Our business plan is that I decide which direction we are going to go, and we do it," he says.

The retail consultants, at least, are optimistic. "Kids and tweens are aspirational," says Cynthia Cohen, president of Strategic Mindshare. "If [Mary-Kate and Ashley] maintain a good, fashionable product, they might be able to carry it into their 20s. Of course, their success at Wal-Mart is tied to their dominance in the media."

The Olsens' onscreen future is less predictable. Their new straight-to-video release, Getting There, is a guaranteed success, but the twins' next feature film, roughly scheduled for 2003--their first since 1995's It Takes Two--is not. Andy Tennant, who directed the girls in It Takes Two, has doubts. "There are no real [adult] twin movies," he says. "Have you ever seen one? They should just go away and come back as one person. A hybrid. They could call her just 'Kate.' "

Beyond retail and the big screen, the ideas get scattered. Globalization is first. The retail line has already hit Wal-Mart affiliates in Britain, and the girls' videos, television shows, and--eerily--publicity photos are being released there chronologically. After Britain, Thorne says, "I want to go into Mexico, Japan, and China with a package, wrap it up in a bow, and say, 'Here's everything at once, ready to go.' Then it becomes Elvis Presley overnight."

Then there's age expansion. ("I think you can accommodate a 30-year-old and a 3-year-old customer. Maybe they'll co-write a book for adults," Thorne says.) And gender expansion. ("Giorgio Armani and Donna Karan sell to both men and women.") Thorne even hired a reporter who had suggested a Mary-Kate and Ashley marriage magazine when the twins hit their late 20s.

It sounds ludicrous, but Thorne has been right before. The twins, Dualstar, and Thorne will continue to be successful at least until the girls head off to college. By then Thorne will have had time to put his next move in place. (Mary-Kate and Ashley as Doublemint Twins?) "Someday they're going to have me come to Harvard Business School to explain how this works," Thorne says. The speech to MBAs might begin something like this: Get 'em while they're young.

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