David Duffield founded PeopleSoft, the company that would put him on the Forbes map, in 1987 at the age of 47. His fourth startup, he mortgaged his house to fund it, but he says he wasn't nervous: "You kind of know when you're on to something."  He sure was: PeopleSoft became the world's second-largest application software company before it was acquired by Oracle in 2005, and Duffield vaulted onto the Forbes 400, with a fortune estimated this year at $1.2 billion.  Young-gun billionaires like Mark Zuckerberg and Sergey Brin have captured the public's attention with their meteoric success, but fortune-building is by no means the preserve of the youthful and brilliant. Many have soared into billionaire-dom later in life after changing careers, bouncing back from losses or making crucial connections. For the past eight years, the average age of new members to the Forbes 400 and Billionaires lists is 54.  Before building PeopleSoft Duffield says he hadn't nailed down the right management style or company ethos, which he came to believe was a focus on customer service and a team atmosphere where employees believe they're valued and "work together in a global spirit." Of his prior businesses, he says, "Frankly, [they] didn't work. [Success] came with time. I made a lot of mistakes along the way."  John Sperling had a wild idea in his 50s that ended up making him wealthy. A humanities professor at San Jose State University, his life changed in 1972 when he was assigned to run a federally funded series of classes to help police and schoolteachers work with juvenile offenders. Sperling was struck by how eager the adults in the program were to further their educations, which they had limited opportunities to do.  At age 53 in 1976, he went out on his own to create a for-profit university, the University of Phoenix. Credited with founding the for-profit education industry in the U.S., Sperling wrote in his autobiography, Rebel With a Cause, "Money I've made notwithstanding, I am an unintentional entrepreneur and an accidental CEO." He says risk-taking was vital: "We violated much that was sacred in the groves of academia."  Eventual billionaire Dietrich Mateschitz took his time before joining the working world, spending 10 years as a college student, bouncing from school to school and working as a ski instructor. "Life as a student is enjoyable," the Austrian told Forbes in 2005. He eventually went into marketing.  Traveling in Asia while working for the German cosmetics company Blendax, he was struck by the popularity of syrupy tonic drinks sold as revitalizing agents. By happenstance a Blendax licensee in Thailand named Chaleo Yoovidhya also owned a tonic drink company. Forming a partnership with Yoovidhya, in 1987 at age 48 Mateschitz set out to sell a reformulated version in the West, called Red Bull.
Some strike it rich young, but for the tycoons on this list, wealth came only after long years of work, failure and chance inspiration.