According to Forbes there are 2,153 billionaires in the world, with a combined net worth of around $8.7 trillion (£6.6tr). Despite sounding like an astonishing number, there are actually 55 fewer people on the list than last year, and a record 46% are, in relative terms, poorer than 12 months ago. Nonetheless there are a number of fresh-faced entrepreneurs under 40 who earn the accolade of being the youngest self-made billionaires on the planet. Here's who they are, from oldest to youngest.