In 2015 demographers, teachers and politicians will stop talking about the population pyramid and start referring to the population dome. The change in terminology will reflect a profound shift in the shape and structure of societies—a shift that has been going on for 50 years and is only half complete.

The pyramid is a traditional way of visualising and explaining the age structure of a society. If you draw a chart with each age group represented by a bar, and each bar ranged one above the other—youngest at the bottom, oldest at the top, and with the sexes separated—you get a simple shape. In 1970 that shape was a pyramid because the largest segment of the global population was the youngest (0-5 years old, comprising 14% of the total), followed by the next-youngest (6-10, with 13%), and so on in regular increments until, above 85 years, there were so few people that the shape vanished into a point (see left-hand chart).

The pyramid was characteristic of human populations since the day organised societies emerged. With lifespans short and mortality rates high, children were always the most numerous group, and old people the least. A population chart of England in 1700 looks like a pyramid, as well.

But now look at the chart of the global population in 2015. It looks more like the dome of the Capitol building in Washington, DC (middle chart). Young children are still the largest group, but now make up only 10% of the population, and those above them are almost as big a cohort, with 9.5%. The age groups start to become markedly smaller only about the age of 40, so the incline starts much further up the chart than with the pyramid. In 1970 the youngest had not only been the largest but also the fastest-growing section of the population. But between 1970 and 2015, the population aged 0-19 grew by only 42%, whereas the population aged 20-39 rose by 128%. This group added almost twice as many people to the overall numbers as the group aged below 20 did. There are now also 50m people above 85, so the dome of 2015 has a spike.

In 1970-2015 the dominating influence on the global population was the fertility rate, the number of children a women would typically bear during her lifetime. It fell dramatically over the period, meaning that the world shifted from having larger to smaller families. But in 2015-60 the biggest influence upon the population will be ageing. Small families are already becoming the norm, the fall in fertility is slowing down and now almost everyone is living longer than their parents—dramatically so in developing countries.

So, by 2060, the dome will have come and gone and now the shape of the population looks more like a column (or perhaps an old-fashioned beehive). It is a little fatter near the bottom and curves in at the top. But up to the age of about 50, the generations are of almost equal size and the shape has near-vertical sides.

The size of the Earth’s population is still rising, from 7.2 billion in 2015 to 9.5 billion in 2060. But, according to calculations by Emi Suzuki and Wolfgang Fengler of the World Bank, two-thirds of the extra 2.2 billion people in 2060 will be in the age group between 40 and 79, not from younger people. The increase in the last, oldest segment is especially marked. Between 2015 and 2060, the number of 60- to 79-year-olds will increase by 1.1 billion, or 131%. That is five times the increase in the number of children and teenagers, which will rise by only 220m, or 9%. The numbers of the oldest people of all (those above 85, here lumped together in one bar) will rise at the fastest rate of all (by 281% in 2015-60), but from a much lower base, so they do not add as many people to the total.

For all of history, humans have lived in societies dominated (in numbers at least) by children. By 2060 children will be barely more numerous than any other age group up to 65. And looking after parents and grandparents will be as big a, or a bigger, social requirement ­as bringing up children and grandchildren. The year 2015 is, roughly, the halfway point in this astounding transformation.

John Parker: environment editor, The Economist