Fifty years ago, in 1976, the median age of the global population was just under 21 years. That means of the 4.1 billion people on Earth at the time, about half were younger than 21 and half were older. Today the median age is 31, and by 2050 the United Nations projects it will reach 36. The typical human being is steadily getting older.

What is the replacement rate?

The engine of that change is fertility. Demographers measure it using the total fertility rate, the average number of children a woman would have over her lifetime at current birth rates. The figure that matters most is the replacement rate, generally put at about 2.1 births per woman. That is the level at which a generation exactly replaces itself, keeping the population stable over the long run without immigration. The slight margin above two accounts for children who do not survive to adulthood.

The global fertility rate today is about 2.2, barely above replacement and down from approximately five in the 1960s. The United Nations expects it to reach the replacement level around the middle of this century and to keep falling after that. More than half of all countries are already below replacement, including China, the United States, India, Japan and most of Europe.

In practical terms, a fertility rate below replacement means that, over time, each generation is smaller than the one before it. Fewer babies today means fewer working-age adults tomorrow, and a growing share of retirees supported by a shrinking workforce. That is the pressure now facing pension systems, health services and labour markets from Italy to South Korea. It is why population ageing, more than raw numbers, is becoming the defining demographic story of the century.