Ask any group of teenagers what they want to be when they grow up. You’ll find a high proportion don’t want to be anything at all: they just want to be extremely rich. They will have found encouragement and discouragement in equal measures in How to be a Young Billionaire (6 April), a documentary about Britons in their twenties trying to make it in the Californian app industry. Mobile phone applications are, on the face of it, a quick way to fabulous riches: something you invent while cleaning your teeth can soon be bringing in millions of pounds a year.It is, of course, not that simple. Turning an app from an idea to a downloadable product takes time and is very expensive. And the odds are overwhelmingly against you. According to this film, 5,000 apps are released every week.