Vienna: The world’s population is growing older as people live longer, and fertility and birth rates fall.

A study, published online by Nature, carried out by the International Institute for Applied Systems Analysis at Laxenburg, Austria, examined population forecasts and fertility rates in 13 major regions of the planet.

The future paths of population ageing result from specific combinations of declining fertility and increasing life expectancies in different parts of the world, it says.

The “speed of ageing is likely to increase over the coming decades and to decelerate in most regions by mid century”.

The study found the overall average of the world’s population will increase from 26.6 years in 2000 to 31.1 in 2050 and then to just 32.9 in 2100, slightly less than what it was in China in 2005, owing to large differences in the regional patterns of ageing.

The researchers say the probability that world population growth will end in this century is 88 per cent, somewhat higher than previously believed. After mid century, lower rates of population growth are likely to coincide with slower rates of ageing.

By the middle of the century, the average Briton will be 48.4 years old, against 39.1 years old now.

But by 2100, the rate of increase will have slowed, with the average age rising to 53.5 years.By the middle of the century it is likely that a third of the population in Britain will be over 60 thanks to people living longer coupled with falling fertility rates

They found that by the middle of the century there is an 82 per cent chance that a third of the population in Britain will be over 60 thanks to people living longer coupled with falling fertility rates, compared with 98 per cent in Japan/Oceania and close to zero per cent for sub-Saharan Africa.