The world’s richest chief executives have all either founded or built companies from the ground up, in many cases transforming their industries. While some of them inherited wealth, most came from modest beginnings. One started as a newspaper boy, another as an insurance salesman, a third as a mortgage broker. Our list of the 10 richest CEOs spans across four generations; the youngest CEO on the list was born in 1984, the oldest in 1930. The ranking is based on net worth figures from Wealth-X, a research firm that provides information on ultra high net worth individuals to private banks and consulting firms. Each chief executive's net worth was calculated as of March 16, 2012 and consists of public and private holdings, estimated cash salaries, dividends, and all other investible assets. Their wealth is so large that it makes the earnings of the average CEO — with their stock options, pension plans and bonuses — look like pocket change. But the recession in 2008 and a slowdown in global growth in recent years have dented the wealth of some of them. So, who are the world’s billionaire-CEOs? Click ahead to find out.