The number of people aged over 65 is on course to overtake the number of children under the age of five for the first time in world history, according to a study published yesterday.

Another 870,000 people turn 65 every month, according to the new figures. Thanks to rising life expectancy, their ranks will soon be growing by almost two million a month and, by 2040, their numbers will have doubled to 1.3 billion.

This will lead to a relative shrinking of the working population and create large new pension costs, which threaten to reduce the overall growth of the world economy, according to analysts.

The US Census Bureau predicts the world will soon have more pensioners than children under the age of five. The lines on the graph will cross within a decade, marking a decisive moment in the greying of the globe.

Richard Suzman, from the National Institute of Ageing in Maryland, which commissioned the study, said: “Global ageing is changing the social and economic nature of the planet.

“The fact that, within 10 years, for the first time in human history, there will be more people 65 and older than children under five in the world, underlines the extent of this change.”

While the populations of Europe and North America have been steadily ageing for decades, this trend is now spreading to the developing world, especially Asia. Rising life expectancy and better health care, coupled with lower fertility rates, mean that some Asian nations must prepare for large rises in their pensioner population.

In China, the number of citizens above 65 will more than treble from 106 million today to 329 million by 2040. This will significantly increase the cost of pensions and impose a major constraint on the future growth of China’s economy.

The proportion of South Koreans over 65 will also treble from 10pc to 29pc, while Singapore will see the most dramatic change, with the pensioner contingent rising from nine per cent today to 33pc in 2040.

Leaving aside Zimbabwe, where an Aids epidemic and general starvation means that most people die before the age of 40, life expectancy is rising strongly almost everywhere.

Consequently, in absolute terms, most people of pensionable age already live in poor countries. “Well over half of the world’s people aged 65 and over now live in developing nations: 62pc, or 313 million people,” said the Census Bureau report. “By 2040, this share is projected to exceed three quarters, with the absolute number of older people in developing countries topping one billion.”