At six months old, most humans are familiarizing themselves with their fingers and toes. The Olsen twins, however, were launching a multi-million-dollar fashion and entertainment empire.

Of course, the twins didn’t start their career by choice. In 1986, Mary-Kate and Ashley’s parents signed up the kids to star in the ABC show Full House. Sure, they were a bit on the young side, but the role of Michelle Tanner -- the youngest and thus most adorable -- was just too good to pass up. Never mind the naysayers who believe you can’t put a price on childhood. As it turns out, there is, in fact, a price.

For the Olsen family, it started at $4,000 an episode. When it became clear that the twins were stealing the show, Jarnette Olsen, the twins’ mom, hired Robert Thorne, a record-industry lawyer, to represent the kids. Soon, the salary shot up to $25,000 an episode. By the series end, the twins were earning $80,000 a show.

Why the sudden uptick in salary? There was just something about the Olsen twins that made them stand out. They weren’t particularly beautiful. They weren’t particularly hilarious or bright. They were just normal. And fans loved it.

In 1990, Parachute Publishing released the first paperback featuring Michelle Tanner. It went on to sell 70,000 copies, and soon Parachute found itself publishing book after book about the twins. They've since published well over 100 Olsen titles.

Next came the movies. Our First Video sold 400,000 copies. To Grandmother’s House We Go followed, earning Mary-Kate and Ashley a mere $125,000.

At this point in their careers, the Olsen twins had more money than any kid could dream of. But it all changed when Thorne created Dualstar, the Olsens’ own production company. Dualstar has released over 30 videos, racking up over $400 million.

Mary-Kate and Ashley assumed control over Dualstar, and have surprised most by running the company both creatively and profitably.

“We've been getting more involved," Ashley told the Los Angeles Times in 2000. "We're old enough now to understand everything and know what's going on and what should be going on."

In the years since then, the Olsen twins have proven that they truly are as adept as any marketing guru or corporate CEO. They created a clothing line sold at Walmart (WMT) called Mary-Kate and Ashley: Real fashion for real girls. In 2006, they became the poster children for Badgley Mischka, an upscale fashion line worn by celebs like Oprah and Sharon Stone. They also have The Row, a two-year-old high-end designer label. And this fall, they launched another clothing line called Elizabeth and James, which includes women's items for prices ranging from $95 to $800 sold in department stores like Bloomingdale's and Neiman Marcus.

By 2005, Dualstar -- which includes all businesses Olsen-related -- was reportedly bringing in $1.2 billion in revenues. No public financial information is available on the private company.

Like any big-named celebrity, the Olsen twins have had to endure tabloid-fueled criticism. In 2006, the National Enquirer printed a headline reading “Ashley Olsen Caught in Drug Scandal.” The story was shown next to an image of Ashley looking, well, totally inebriated. Olsen denied the story and filed a $40 million lawsuit against the magazine.

While the Olsen twins may be one of the most successful teams of female entrepreneurs out there, most fans still remember the days when Mary-Kate and Ashley played the innocent and adorable Michelle Tanner.

Just recently, VH1 named the Olsen twins number-three in their list of the top 100 greatest child stars.

Number one? Gary Coleman, who in 1999 declared bankruptcy.

Meanwhile, the Olsen twins are worth an estimated $100 million. Not bad for a couple of 23-year-olds.