Experts on ageing are divided as to where these limits lie and how far life expectancy can be extended.

However the consequences for everyday life in what French philosopher Alfred Sauvy has described as "the century of ageing" will almost certainly be no joke.

At the present time, according to UN population figures, the world has 600 million people aged 60 or over. By 2050 the figure is expected to reach two billion. Then, for the first time in history, the over-60s will outnumber the under-14s.

The implications for pensions, work, healthcare, family life and much else are enormous.

With a proportionally reduced workforce required to work harder to support ever more pensioners seeking greater benefits, new conflicts will arise.

As financial writer Hamish McRae has predicted in the London daily The Independent, political tensions henceforth will not so much be between left and right as between young and old.

Meanwhile futurologists, extrapolating from the vertiginous rise in life expectancy in the 20th century, are predicting that lifespan, boosted by medical developments such as tissue fabrication and gene therapy, could rise as high as 130.

Such optimism is dismissed by scientists and doctors, at least in the short to medium term.

"We will continue to edge upwards, but for life expectancy to increase to 130 years would require quite astonishing advances in the treatment of so many diseases," says Tom Kirkwood, professor of gerontology at Newcastle University. "Once you get to old age there are multiple causes of death. If you manage to push one back, there's always another waiting in the wings."

The big advances in life expectancy have already been made, he points out. Even if death rates from heart disease and cancer, today's major killers, could be dramatically reduced, this would only add five to 10 years. Such a gain would nevertheless be well worth having, "not least because we might hope to improve the 'healthspan' even more than we improve the lifespan."

Kirkwood does however believe that "ageing is neither inevitable nor necessary." There is no such thing as a "death gene," he argues. The human body is "programmed to survive, but not programmed well enough to last indefinitely."

He agrees with Michel Allard of the Fondation Ipsen in Paris, who argues that there is no limit in sight to the present growth in life expectancy.

Since the recent improvement is clearly not due to genetic changes, it must derive from environmental factors which now permit us much more than before to fully express

our genetic heritage, Allard says.

For Allard, the case of Jeanne Calment, the Frenchwoman who died in 1997 aged 122, establishes that there is a theoretical potential of at least 120 years.

Citing research by colleague James Vaupel, he believes that half of French girl babies currently being born could live to become centenarians.

According to Vaupel, even if current death rates show no further improvement, more than half of all French babies will live to exceed 80, and half of all girl babies will live beyond 85.

Allard warns however that social structures will have to adjust radically to cope with the demands of a generation willing and able to remain active as it grows older, for example inventing new professional cycles that allow for retraining and part-time work beyond conventional retirement ages.

Only half-joking, he predicts a re-run of the May 1968 student riots in Paris as the postwar "baby-boomers", 50 years further on in May 2018, take to the streets to protest at the idleness enforced on them in their seventies.

Experts differ as to how far longer lives will mean healthier lives. The issue at stake is the burden that extended periods of disability could place on society if improvements in lifespan are not matched by improvements in health standards.

Recent surveys in the United States suggest a significant decline in the prevalence of disability among older people, age for age, between 1982 and 1994.

And Kirkwood for one is upbeat about the prospects for longer, healthier lives.

"The social costs are challenging but the danger has often been exaggerated," he says. "If older people can maintain independence and perhaps the means to provide some income for longer, the costs need not increase as dramatically has some have feared."

The first requirement, however, "is to reexamine our whole concept of later life in a positive way."

(China Daily)