Image caption Advances in nutrition and medicine have had an impact on how long we are living healthy lives

Move over Methuselah. Future generations could be living well into their second century and still doing sudoku, if life expectancy predictions are to be believed.

Increasing by two years every decade, they show no signs of flattening out. Average lifespan around the world is already double what it was 200 years ago.

Since the 1980s, experts thought the increase in life expectancy would slow down and then stop, but forecasters have repeatedly been proved wrong.

So can we go on living longer and longer? Is there a limit to how long we can survive into old age?

The reason behind the steady rise in life expectancy is "the decline in the death rate of the elderly", says Professor Tom Kirkwood from the Institute of Ageing and Health at Newcastle University.

He has a theory that our bodies are evolving to maintain and repair themselves better and our genes are investing in this process to put off the damage which will eventually lead to death.

As a result, there is no ceiling imposed by the realities of the ageing process.

"There is no use-by-date when we age, ageing is not a fixed biological process," Professor Kirkwood says.

A large study of people aged 85 and over in Newcastle, carried out by Professor Kirkwood and his colleagues, discovered that there were a remarkable number of people enjoying good health and independence in their late 80s and beyond.

We are reaching old age with less accumulative damage Professor Tom Kirkwood, Newcastle University

With people reaching old age in better shape, it is safe to assume that this is all down to better eating habits, living conditions, education and medicine.

There are still many people who suffer from major health problems, but modern medicine means doctors are better at managing long-term health conditions like diabetes, high blood pressure and heart disease.

"We are reaching old age with less accumulative damage than previous generations. We are less damaged," says Professor Kirkwood.

Our softer lives and the improvements in nutrition and healthcare have had a direct impact on longevity.

'No natural limit'

Nearly one-in-five people currently in the UK will live to see their 100th birthday, the Office for National Statistics predicted last year.

Life expectancy at birth has continued to increase in the UK - from 73.4 years for men for the period 1991 to 1993 to 77.85 years for 2007 to 2009.

Life expectancy for females at birth has also increased - from 78.9 years (91-93) to 82 years (2007-2009).

A report in Science from 2002 which looked at life expectancy patterns in different countries since 1840, concluded that there was no sign of a natural limit to life.

Researchers Jim Oeppen and Dr James Vaupel found that people in the country with the highest life expectancy would live to an average age of 100 in about six decades.

But they stopped short of predicting anything more.

"This is far from eternity: modest annual increments in life expectancy will never lead to immortality," the researchers said.

There has been no flattening out of life expectancy Prof David Leon, LSHTM

"It is striking, however, that centenarians may become commonplace within the lifetimes of people living today."

Gene play

We do not seem to be approaching anything like the limits of life expectancy, says Professor David Leon from the London School of Hygiene and Tropical Medicine.

"There has been no flattening out of the best of the best - the groups which everyone knows have good life expectancy and low mortality," he says.

These groups, which tend to be in the higher social and economic groups in society, can live for several years longer than people in lower social groups, prompting calls for an end to inequalities within societies.

Within populations, genes also have an important role to play in determining how long we could survive for - but environment is still the most important factor.

It is no surprise that healthy-eating, healthy-living societies like Japan have the highest life expectancies in the world.

But it would still be incredible to think that life expectancy could go on rising forever.

"I would bet there will be further increases in life expectancy and then it will probably begin to slow," says Professor Kirkwood, "but we just don't know."

Methuselah is not turning in his grave just yet.