THE 1960s WERE a great decade for anthems of generational takeover. Songs like “The Times They are a-Changin’” by Bob Dylan, “My Generation” by The Who and “Five to One” by the Doors did not just rail against the middle-aged and old—they actually predicted their ousting. The singers were confident about this partly because of demography. America was a young country in 1965, with a median age of 28, and was getting younger. As Jim Morrison put it in “Five to One”: “They got the guns/But we got the numbers.”

Well, the times they have a-changed. The median American is now 38, the median non-Hispanic white American six years older than that. The world is rapidly going the same way. Although some countries are still extremely youthful—the median age in Niger is 15—almost all are ageing. Humanity as a whole is already older than America was in the mid-1960s. In 2020, for the first time in history, the median person will be older than 30.

Two forces are driving this change. The first, bigger one is overall demographic ageing. As the global birth rate falls and lifespans lengthen, the proportion of older people in the population is growing. The second cause is that a great many people have just crossed from their late 20s into their early 30s. According to the UN’s demographers, in the past decade no five-year age group has grown faster than 30- to 34-year-olds, whose numbers have swelled from 506m to 606m. More than half of the additional early-30-somethings are Chinese and Indian. The huge cohort born in the 1980s, which flocked to cities in the early years of this century, powering Asia’s industrial development, is now settling down and worrying about school fees.

One result is that the world will have fewer energetic workers. The global population of 20-somethings is barely growing, and if you exclude Africa—where, sadly, most young people cannot find decent jobs—it is shrinking. Firms will respond by automating. A paper by two American economists, Daron Acemoglu and Pascual Restrepo, finds that countries with relatively old workforces (like Germany and Japan) rely on industrial robots more than younger countries (like America) do.

You say you want a revolution?

Politics will change, too. Protest-marchers and barricade-builders are disproportionately young and childless. Although street protests can sway governments in aged countries, such as France and Ukraine, they are more potent in young ones. The Arab spring of 2011 was attributed partly to a bulge of young, underemployed people in north Africa and the Middle East. But that region is ageing, with some countries already beyond their most rebellious years. The median Iranian is now 32, the median Tunisian 33. They could hit the streets, but they will have to find babysitters first.

In mature democracies, by contrast, politics could become more unstable. Despite a reputation for zealotry, young people in the West are often more moderate than their elders. Opinion polls by Pew, a think-tank, show that 55% of millennials in America call themselves political moderates or centrists. Every other cohort—Generation X, baby-boomers, the elderly “silent generation”—is more extreme, generally in a right-wing direction. It was old voters, not young ones, who put a human wrecking-ball in the White House and blew up Britain’s constitutional order by voting for Brexit in 2016.

This white-haired rebellion may well continue. As countries age, they will need more youthful workers to keep their businesses humming and to pay for pensions. The obvious way of getting them is by letting in more immigrants. But old people are often violently opposed to that, and will punish governments that do it. To rephrase Morrison: they got the guns, and the numbers too.

This article appeared in the International section of the print edition under the headline “Forever middle-aged”