Policymakers fretting over the costs of caring for the aged will face their greatest challenge in the next two decades, although the burden should ease towards the end of the century.
So says a study which suggests that the much-feared crisis of ageing of the world's population is already starting to bite, as the post-World War II Baby Boomers shuffle into retirement, inflicting a heavy hit on budgets in developed countries.
The percentage of greyheads in the world's population will increase rapidly over the next 20 years, although this acceleration will peak in different regions at different times.
Japan is already nearing the period when the proportion of ageing people in its population is starting to quickly surge.
North America, Europe, China and the countries of the former Soviet Union will follow sometime between 2020 and 2030, according to the paper, published on Sunday by the British journal Nature.
South Asia will go through a decade of rapid ageing beginning in 2035, the Middle East in 2040, and sub-Saharan Africa at mid-century.
"It is really important for policymakers to take these figures into account," said one of the authors, Warren Sanderson of the State University of New York at Stony Brook.
Most demographic projections fail to acknowledge nuances about how quickly a population ages, but the implications are far-reaching for healthcare, where costs rise dramatically towards the end of life, he said.
At present, 10 percent of the world's population is over 60; this will slowly rise to 13 percent by 2020 but leap to 17 percent by 2030.
After than it will continue to climb gradually to 32 percent by 2100.
In China, the increase will quadruple, from 10 percent today to 42 percent by 2100, whereas nearly half the population in Western Europe -- 46 percent -- will be over 60 by century's end, compared with 20 percent today, according to the study.
By these yardsticks, future generations would appear to be doomed to carrying a huge social burden.
"People look at those numbers and they get very scared, thinking that their healthcare expenditures are going to explode," said Sanderson.
But, he said, the biggest challenge will fall before 2030 for developed countries and by 2050 elsewhere, because so many people will age so quickly in the coming years.
Paradoxically, the health-cost crisis could be relatively easier to manage in the decades after that.
Despite the larger proportion of elderly, many of the over 60s in the latter half of this century are likely to be hale and hearty and will not need to go to the doctor. Medical care becomes most expensive in the last few years of life, and not before.
Some studies have calculated that someone who is 65 in 2100 can expect to live beyond 90.
"There are two ways to look at age: one is how many birthdays you have already had, and the other is how many you expect to have in the future," Sanderson explained.
Instead of looking at the percentage of people aged over 60 -- today's typical benchmark for old age -- we should look at the percentage of people whose life expectancy is 15 years or less. By 2100, nearly a third will be aged over 16, but only 16 percent will be in this final, costly stage.
"Looked at this way, the figures are rather reassuring," he said.