The world's population is set to exceed nine billion people by 2050 but almost a quarter of them will be over 60, a United Nations report has revealed.
The growth will come mainly in the developing world, with India set to pass China as the most populous nation.
But the population in rich nations is expected to stay broadly the same, with increased migration compensating for falling birth-rates.
And the number of elderly people is expected to almost treble by 2050.
The report, by the United Nations Population Division (UNPD), predicts that over the next 43 years the global population will increase from the current 6.7 billion to 9.2 billion.
This increase, it says, is equivalent to the total world population in 1950.
In less developed regions, the population is expected to rise from 5.4 billion to 7.9 billion.
India, with 1.65 billion people, will be the most populous nation, followed by China, the US, Indonesia, Pakistan and Nigeria. In countries such as DR Congo, Afghanistan and Ethiopia, populations are expected to more than double.
There are a number of variables. The figures could be even higher, the study says, because the predictions are based on improved access to family planning across the developing world.
But they could also be lower, if the provision of anti-retroviral drugs to HIV-Aids patients does not increase as predicted.
In the developed world it is a different picture, with falling birth rates and emigration causing populations of some countries to shrink.
Japan, for example, where the birth rate is already an issue of considerable concern, is expected to see its population fall by more than 20% by 2050.
In Eastern Europe, sharp falls are forecast in countries such as Bulgaria and Poland, triggered in part by the movement of workers overseas.
Migration from the developing world is expected to plug some of the gaps.
The study predicts that net migration from developing to developed countries will average 2.3 million people every year.
The number of people over 60, meanwhile, is predicted to soar from 673 million in 2005 to two billion by 2050, an increase attributed to improved life expectancies.
With the majority of births in the developing world, the proportions of old and young people in wealthy nations will shift, potentially leaving gaps in labour forces and countries struggling to care for their elderly.
"Population aging is in fact the result of a success - the success of humanity in controlling its number," said Hania Zlotnik, director of the UNPD.
"The only thing we can hope is that the aging continues and that society can adapt itself to the important social changes."