At the tender age of 14, Jessica Mah learned a tough lesson about business. Her first company, which rented server space to small and medium-size businesses, had grown quickly to 1,200 customers and $6,000 a month in revenue. Rather than renting server space, as she had done in the past, Mah began to purchase hardware. Inexperienced at managing cash flow, she found it difficult to keep up with expenses. "I started the company with $10 and a homemade desktop computer, and somehow that initial $10 ended up in me spending $10,000 a month in upkeep and investments to grow the company," Mah recalls. Feeling overwhelmed, she sold the business at a marginal profit.

Mah then set up and began to blog about business and technology from a young person's perspective. (Sample headline: "Idiots do nothing. Entrepreneurs sometimes feel like idiots. Both sound like me!") The blog quickly drew more than 100,000 unique visitors a month. Last fall, Mah, now a Berkeley junior, came up with the idea for a new venture, which she began to chronicle on her blog. "All of us were looking for internships, and I had no clue where to start," Mah says. "This is how business ideas begin: You want to do something, and you run into problems."

Mah ran her concept for a website to help students find internships, primarily at tech start-ups, past her friends Arielle Scott and Andy Su. The idea was to help employers and potential interns cut through help-wanted listings by incorporating what Mah calls "a nimble version of the Amazon relevancy algorithm." Su and Scott liked the concept and signed on to help Mah launch InternshipIN. Equal parts Craigslist and Mediabistro, InternshipIN helps students identify high-quality internships -- i.e., internships that do not involve "getting coffee and making copies," Mah says. Much of the initial traffic has come through a partnership with SimplyHired, a job search engine. Eventually, Mah hopes to begin charging employers a fee to post listings on the site.

Though many find that running a start-up by committee is tough, Mah believes that collaborating helps to make a business stronger. In fact, she ran her first business with two other teenagers -- including one from Australia with whom she communicated via webcam. "You always hear about Bill Gates and Steve Jobs, but Bill Gates had Paul Allen, and Steve Jobs had Steve Wozniak," Mah says. "If you have co-founders and don't try to make it a one-man operation, things work great."

One thing Mah has changed: With her first business, she maintained a "zero disclosure of age" rule, which backfired when a few clients discovered the truth and fled. This time around, Mah has found that being the very public face of her business is fun and has its advantages. Her blog and InternshipIN scored her a coveted invitation to the TED Conference, an exclusive gathering of innovators and technologists. She felt right at home. "Once you're an entrepreneur, there is no turning back," she says. "The reason is you're working on something you believe in, you're actually solving a problem, you're making money in the process, and you're learning something new every day."