These national patterns mean it does not make sense to look only at overall trends, such as the size of a country’s population or its growth rate. Population is changing within national borders, too, and this means that different generations are changing in size, with some age groups, notably economically-active adults changing relative to others. If you look at these disaggregated trends, the world divides itself into three categories: young, middle-aged and old.
First come the young countries, where the working-age population is rising relative to everyone else and where the median age stays below 40. This is most of sub-Saharan Africa, India and the Middle East. Then group will reap a so-called demographic dividend because they will have a lot of adult workers relative to the numbers of dependent old people and children (though to reap this dividend they will have to get their policies right: merely having a large number of young adults is not enough by itself).
Next comes a group of countries which might be called middle-aged or the in-betweeners: not as young as Africa and India, not ageing as fast as Europe. They include the United States, most of Latin America, and South East Asia. In this group, the proportion of dependent children and old people will rise, but not a lot, and societies will get older, but the median age will still be in the mid to late 40s.
They will face problems of ageing and of providing workers for everything from productive companies to the army. But these problems will be as nothing compared with the real ageing countries, who are the big losers from the patterns of demographic change: Europe, Japan and China.
By 2050, Japan will have almost as many dependants as working-age adults. No society has seen such a thing before. And China is ageing at an unprecedented rate. In 1980, its median age was 22. In 2020, it will be 38, older than the United States. In 2040, it will be 47, older than Europe. These demographic trends will be a big challenge not only to its continued economic growth but also to its chances of challenging the United States geopolitically, since they will make it harder and more expensive to fill military ranks and modernise the army.
John Parker is the Globalization editor of The Economist.