YOU MAY KNOW that the world’s population is aging — that the number of older people is expanding faster than the number of young — but you probably don’t realize how fast this is happening. Right now, the world is evenly divided between those under 28 and those over 28. By midcentury, the median age will have risen to 40. Demographers also use another measure, in addition to median age, to determine whether populations are aging: “elder share.” If the share, or proportion, of people over 60 (or sometimes 65) is growing, the population is aging. By that yardstick too, the world is quickly becoming older. Pick any age cohort above the median age of 28 and you’ll find its share of the global population rising faster than that of any segment below the median. By 2018, 65-year-olds, for example, will outnumber those under 5 — a historic first. In 2050, developed countries are on track to have half as many people under 15 as they do over 60. In short, the age mix of the world is turning upside down and at unprecedented rates.

This means profound change in nearly every important relationship we have — as family members, neighbors, citizens of nations and the world. Aging populations also alter how business is done everywhere. The globalization of the economy is accelerating because the world is rapidly aging, and at the same time the pace of global aging is quickened by the speed and scope of globalization. These intertwined dynamics also bear on the international competition for wealth and power. The high costs of keeping our aging population healthy and out of poverty has caused the United States and other rich democracies to lose their economic and political footing. Countries on the rise amass wealth and geopolitical clout by refusing to bear those costs. Older countries lose work to younger countries.

To see this process at work, look at China. In its march to prosperity, the country has encouraged hundreds of millions of its young people to move into cities. Chinese metropolises — some, like Beijing, ancient but newly sprawling, others, like Shenzen, built from scratch — are where the factories are. Foxconn Technology Group, for example, the giant electronics manufacturer that builds components for Dell, Hewlett-Packard and Apple in gigantic plants in Shenzen and elsewhere in urban China, will soon employ enough people to fill 60 percent of the jobs in Manhattan. Foxconn has close to 920,000 workers, nearly all of whom are under 25; in August, the company announced plans to add 400,000 more workers in the next year. But China’s is a kind of Dorian Gray economy, its young and footloose global identity hiding a grayer reality. By and large, older workers have been excluded from its remade, globalized economy. They are left behind in their rural villages, or they are pushed from their urban homes into the ghettos of dour apartment blocks on the urban edge to make room for the new apartments and offices occupied by younger urbanites and the companies eager to hire them. Discrimination — “age apartheid” might be a better term — is one way to describe what’s going on here: no country sorts its population more ruthlessly by age.