Everybody knows where the population explosion came from. Two centuries ago birth rates and death rates were high everywhere, and population growth was very slow. Then clean water, good food and antibiotics radically cut the death rate—and the human population of this planet increased 300 percent in the past 90 years.

Eventually, as people moved into the cities and big families were no longer an advantage, the birth rate dropped too. The world’s population is still growing, but it will only increase by 50 percent in the next 90 years. So far, so obvious. But what’s happening to the human lifespan is equally dramatic.

Here’s the key statistic: the average human lifespan in a developed country has been increasing at three months per year ever since the year 1840.

Everybody assumes that lifespan grew much faster in the 19th and early 20th centuries, and is growing much slower now. But no. It has plodded along at the same rate, adding about three months to people’s life spans every year, for the past 175 years. And yes, that does mean that a baby born four years from now can expect to live, on average, a whole year longer than a baby born this year.

There have always been some people who lived to 70 or 80, but the average age at death in 1840 was only 40 years. By the year 2000 it was 80 years. That’s 40 more years of life per person in 160 years.

And lifespan is still increasing at the same rate. In Britain, for example, the average lifespan has increased by 11 more years in the past 44 years. Three months per year, just like in the 19th century.

This is why actuaries predict that babies born in the year 2000 will have an average lifespan of 100 years. Give those babies the 80 years of life that people who died in 2000 enjoyed, then give them an extra three months for every one of those 80 years—and they will have 20 years more years to live. That is, an average of 100 years.

This sounds so outlandish that you instinctively feel there must be something wrong with it, and maybe there is. The fact that it has gone on like this for 175 years doesn’t necessarily mean that it will go on forever. But it’s not stopping or even slowing, so the smart money says that it will continue for quite a while yet

What about the developing world? Most of it has been playing catch-up, and by now the gap isn’t very big any more. In China the average lifespan was only 42 years as recently as 1950—but then it began increasing by six months per year, so that the average Chinese citizen can now expect to live to 75. Once you hit an average lifespan of 75 years, however, the pace slows down to three months per year, the same as in the developed countries.

India is a little behind China: average lifespan was still 42 years in 1960, and is now 68, so it’s still going up at six months per year. But we may expect to see it fall to the normal three months per years in about 2030, after the average Indian lifespan reaches 75.

All the developing countries of Asia, Latin America and the Middle East are in the same zone. The sole exception is Africa: where 35 countries have average life spans of 63 years or lower. But even most African countries are seeing a slow growth in average lifespan.

So do we end up with a huge population of people so old they can barely hold their heads up, let alone eat solid food? Probably not.

Three hundred years ago Jonathan Swift wrote about people like that in his satire Gulliver’s Travels. Struldbrugs, he called them: people who could not die, but went on ageing until they were so decrepit and disabled that death would have been a mercy.

They were declared legally dead when they reached eighty, as otherwise their longevity would mean they ended up owning everything. But they weren’t really dead; now it was the public that had to support them for the rest of their interminable lives.

In real life, crippling diseases and disabilities are still mainly a phenomenon of the last decade of life, and as the lifespan lengthens that final decade also moves.

Demographers now talk about the “young old”, who are in their 70s and 80s and still in reasonably good shape—and the “old old”, in their 90s and 100s, who are mostly frail and in need of care. So the time is probably coming when people must work until into their 80s, because the over-65s will amount to a third of the population. No society can afford to support so many.

But by then people won't be decrepit in their 80s. And the only alternative is dying younger.