Joel Kotkin of Pepperdine University in Los Angeles, author of a forthcoming book on the geography of the information age, sees the favourite cities of the Internet economy, such as San Francisco and Seattle, as boutiques for the young, single and rich.

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We humans are an "information economy" and middle age is the time when we pass on most of that information - this is why middle-aged people like being listened to.

BBC: Viewpoint: Just what is middle age?