It suggests that today's 5.8 billion population will increase to 7.9 billion in 2020, rising to 10 billion in 2050. But the proportion of over- 60s is predicted to soar, doubling from 9.5 per cent to 20 per cent in 2050 and increasing further to 27 per cent by 2100.
Wolfgang Lutz and colleagues from the International Institute of Applied Systems Analysis in Laxenburg, Austria, believe that ageing will replace population growth as a focus for public, political and scientific concern. The researchers based their results on an analysis of trends in fertility, mortality and migration.
One of the key elements was a global trend towards low fertility, even in countries of traditionally high birth rate.
At the other end of the equation there was uncertainty about how much longer people will live, with some experts believing the human race to be already close to the biological limit of life expectancy.