IN THE 20th century the planet’s population doubled twice. It will not double even once in the current century, because birth rates in much of the world have declined steeply. But the number of people over 65 is set to double within just 25 years. This shift in the structure of the population is not as momentous as the expansion that came before. But it is more than enough to reshape the world economy.

According to the UN’s population projections, the standard source for demographic estimates, there are around 600m people aged 65 or older alive today. That is in itself remarkable; the author Fred Pearce claims it is possible that half of all the humans who have ever been over 65 are alive today. But as a share of the total population, at 8%, it is not that different to what it was a few decades ago.

By 2035, however, more than 1.1 billion people—13% of the population—will be above the age of 65. This is a natural corollary of the dropping birth rates that are slowing overall population growth; they mean there are proportionally fewer young people around. The “old-age dependency ratio”—the ratio of old people to those of working age—will grow even faster. In 2010 the world had 16 people aged 65 and over for every 100 adults between the ages of 25 and 64, almost the same ratio it had in 1980. By 2035 the UN expects that number to have risen to 26.

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