Four young entrepreneurs at different stages in their careers offer advice on getting over those first hurdles.

One of the advantages of being a young entrepreneur is that you don't know what can't be done. You get to look at the world with an unencumbered perspective and take your own shot at changing it. You don't have to be burdened by old beliefs. Be aware, however, that you may just be repeating mistakes that others have already made (see BusinessWeek.com, 9/14/06, "Some Brilliant Mistakes"). And the world may not give you the benefit of the doubt because you are young—in fact, you are likely to face challenges solely because of your age.

When I decided to write this column about the stumbling blocks that young entrepreneurs face when starting businesses, I spoke to four young entrepreneurs at different stages in their careers to get their advice.

1. Learn how to handle rejection and be persistent.

My friend Jud Bowman, 25, says he had to choose between a scholarship to Stanford and pursuing his dream of creating a mobile search-engine business. Bowman decided to follow his dream and launched Pinpoint Networks when he was 18.

He says that even if you possess incredible intelligence, a track record of success, contagious passion, and an ability to make it seem like you're more mature than you are, young entrepreneurs should prepare to face rejection and doubt. He recalls how his parents repeatedly told him he was crazy to turn down Stanford to start his venture. He says nearly 60 venture-capital firms rejected him before he found an investor, customers wouldn't buy his product at first, and prospective hires wouldn't accept job offers. He remembers that he couldn't even buy a bottle of wine when he took his sales prospects out to dinner.

But he kept at it. He says that Motricity (created after Pinpoint merged with Power by Hand), where he now works as chief technology officer, will deliver $500 million worth of mobile content this year and is positioned to be the dominant player in the mobile applications market.

2. Get ready to take advantage of any opportunity.

One of my current students, Kareem Lee, 22, echoes Bowman's advice. While he says it was easy for him to bootstrap his tutoring and lawn-mowing businesses as a teenager, when he tried pursuing his dream of setting up a record label at the age of 19 in his hometown of Indian