This year Jessica Mah, CEO of inDinero, a maker of online money-management tools for small businesses, moved her company into a new office on the 9th floor of a downtown San Francisco high rise. She fired an employee, and hired a customer service manager. She sometimes worked 80 hours a week and ate dinner her office. And this year, Mah, for the first time in her life, legally drank alcohol.

Mah, who had has founded three start-ups and graduated from the University of California-Berkeley at age 19, turned 21 years old in May.

While at Berkeley studying computer science, Mah and inDinero co-founder/roommate Andy Su, who is now 20, built the prototype for their start-up, which has been called the for small business. She'd been a Mint user for about two years, but says it just didn't work for her business needs.

"When you're managing your business finances, you need more specific business applications," she says. "We thought we needed something right in the smack middle of Quickbooks and Mint."

So she and Su built it: an online dashboard on which small business owners can manage all of their clients, inventory, and accounting. They applied to Y Combinator, and she became a Paul Graham favorite. By Demo Day at the end of the pair's residence at the start-up incubator, Mah and Su had already secured a comfortable half-a-million in funding. After presenting their idea formally to investors, 20 signed on, including Dave McClure, Steve Blank, and Intuit's David Wu.

Finding clients was easy: The slumped economy fostered creation of the smallest businesses, such as individuals selling items on eBay and Etsy, who needed to keep track of myriad small payments with slight margins. InDinero charges small businesses with more than 50 transactions and fewer than 500 per month $29.95 per month; for unlimited monthly transactions, the cost is $99.95 per month.

"Our first paying customer was actually an Etsy seller. They sold custom decals on Etsy, so they'd get a dozen or so sales a day," Mah says. "They tried everything out there, and couldn’t get hooked on difficult-to-use software that tracked so much more complex stuff than they needed to."

Building a user-base is another issue. The six-person company has tried paid search, and SEO, but buzz comes mostly through low-key public relations and word-of-mouth.

This summer, inDinero is launching a mobile app, and Mah admits she’s lost sleep pondering her company's next project.

For now, with a long runway left on their investment, and a shiny new office in a Market Street high rise (with a gym, view, and rooftop deck), the inDinero team is mostly happy to be young and turning what was once an inconvenience—accounting programs—into a business helping other small businesses grow and track that growth.