Kevin Hagen, Contributing writer

Young entrepreneurs who became millionaires in their 20s and 30s seem to cross over all lines of business and come from all walks of life and educational backgrounds.

What sets them apart is a keen ability to see an opportunity and take advantage of it. That, along with a lot of hard work, dedication, determination, self-confidence and a willingness to take risks enable these entrepreneurs to not only succeed, but also to make it big.

Another key to success is service and how the businesses started by these young entrepreneurs have directly affected people's lives. They saw a need and filled it.

Michael Dell saw a need for custom-made computers and was running a $6 million business while still attending school.

Mark Zuckerberg saw a need for a social networking site. By the time he was 23, he had a net worth of $1.5 billion according to Forbes magazine with 66 million active users for his Facebook.

Mark Cuban, a former bartender, launched an early Internet audio and video portal called Broadcast.com, which he sold to Yahoo in 1999 for $5.7 billion. He then diversified his wealth and is the owner of the NBA's Dallas Mavericks and is chairman of HDNet, an HDTV network.

Liz Lange saw a need for sophisticated maternity wear. When prospective retailers wouldn't buy her products, she sold made-to-order clothing by appointment, according to Entrepreneur magazine. With word-of-mouth advertising, Lange grew her business to over $3 million in sales by 2001. Reporting growth in huge multiples since then, she now has three flagship boutiques and has the exclusive maternity line at all Target stores.

When her daughter was born in Aspen in 1995, Julie Aigner-Clark could not find products for her baby that reflected her passion for art and classical music, according to Entrepreneur magazine. She created a video called "Baby Einstein" with captivating pictures and mothers speaking different languages. Aigner-Clark persevered through two years without any interest in her product until she got a break at the American International Toy Fair in New York. By 2000, her company had $10 million in sales, and in 2001 her Baby Einstein company was acquired by The Walt Disney Co.

As a diabetic child, Joseph Semprevivo wanted to be able to eat ice cream. So with some help from his parents, who owned a restaurant, he created a rich, sugar-free ice cream and started marketing it to grocery stores and ice cream shops in New Mexico. When he was 17, Semprevivo began concentrating on sugar-free baked goods using all natural ingredients. By 2005, Joseph's Lite Cookies had over $100 million in projected sales according to Entrepreneur magazine. His product line is now sold in 37 countries and nearly 125,000 stores in America.