Dave Samuel started young. The 33-year-old president of Grouper, a media file-sharing company, and chairman of Brondell, a high-tech toilet maker, owned a banner-printing business by the time he was 11. He also delivered the morning paper, an occupation that he believes most successful entrepreneurs tend to begin their careers with.

When he was in seventh grade, he freelanced as a disc jockey. But his passion for music didn’t just help him make money through high school and college—it helped him make millions before he turned 30.

As he tried his hand at assorted moneymaking ventures while growing up in Maine, Idaho, Mr. Samuel realized he enjoyed doing things on his own. And his grandfather, Lowell Samuel, a clarinet player in the U.S. Air Force band who went on to start a music business, mentored him.

Despite middling SAT scores, Mr. Samuel applied to the Massachusetts Institute of Technology. He was accepted, he believes, only because the admissions committee realized how determined he was to create and work at companies he believed in.

MIT was right. In the past decade, Mr. Samuel has created three companies, starting with an Internet radio company, Spinner.com, in 1996. He and his partner, Josh Felser, sold it to AOL for $320 million in 1999. A year later, he went on a three-year sabbatical: he traveled, got married, and his wife had a baby. In 2003, he founded Brondell, a company that makes a high-tech toilet complete with a warm-water rinse and a blow dry.

Last year, Mr. Felser devised the idea of a program to facilitate peer-to-peer (P2P) sharing of media files. The two teamed up again and launched Grouper in October 2004. Grouper has raised $3.5 million to date from angel investors, the founders, and venture capital firm Duff, Ackerman & Goodrich. The Mill Valley, California-based company says that about a million people have downloaded the program so far.

But startups are unpredictable, and Mr. Samuel remembers how he often thought Spinner might not make it. “I’ve given numerous pitches, and advice to other entrepreneurs,” he says. “The biggest one is that a startup is like a roller coaster—you need to be ready to handle the biggest highs and the biggest lows.”