The world is on the cusp of a staggering rise in the number of old people, and they will live longer than ever before. Over the next 20 years the global population of those aged 65 or more will almost double, from 600m to 1.1 billion. The experience of the 20th century, when greater longevity translated into more years in retirement rather than more years at work, has persuaded many observers that this shift will lead to slower economic growth and “secular stagnation”, while the swelling ranks of pensioners will bust government budgets.