Most babies born in the past few years in the U.K. will live to be 100 if current trends continue, experts said on Friday.

And people could be living not only longer, but better, according to doctors writing in the Lancet medical journal, who say that most evidence shows the under-85s are tending to remain more capable and mobile than before. They have more chronic illnesses, such as cancers and heart conditions, but people survive them because they are diagnosed earlier and get better treatment.

Professor Kaare Christensen and colleagues at the ageing research centre at the University of Southern Denmark calculate that at least half the babies born in the U.K. in the year 2000 will reach their 100th birthday. Life expectancy is increasing so fast that half the babies born in 2007 will live to be at least 103, while half the Japanese babies born in the same year will reach the age of 107.

The bad news is that the ageing populations of rich countries such as the U.K. threaten to unbalance the population. It “poses severe challenges for the traditional social welfare state,” write Christensen and colleagues.

But they have a radical solution: young and old should work fewer hours a week. Over a lifetime, we would all spend the same total amount of time at work as we do now, but spread out over the years.

“The 20th century was a century of redistribution of income. The 21st century could be a century of redistribution of work,” they write. “Redistribution would spread work more evenly across populations and over the ages of life. Individuals could combine work, education, leisure and child rearing in varying amounts at different ages.” It is a theory that is beginning to receive “some preliminary attention”, the authors say, citing a study in the Science journal three years ago which suggested that shorter working weeks would help young people and increase western Europe’s flagging birth rate.

Shorter working weeks might further increase health and life expectancy, Christensen and colleagues write. But redistribution of work will not solve all the problems caused by a society with a large number of very old people. Beyond a certain point, the old will need younger people to look after them — although technology is likely to provide some help in advanced countries such as the U.K.

Over the 20th century there have been huge increases in life expectancy — more than 30 years — in most developed countries.

Breakthroughs in saving babies from infectious diseases and mothers from the complications of childbirth were responsible for the big increases in life expectancy until the 1920s. Then people started to live to greater ages. “This reduction in old-age mortality was unprecedented and unexpected,” the authors write. “Since the 1950s, and especially since the 1970s, mortality at ages 80 years and older has continued to fall, in some countries even at an accelerating pace.” Japan holds the record. In 2007, the life expectancy of a woman was 86 years — confounding theorists who had suggested in 1980 that 85 years was the limit for human life expectancy.

The Danish authors say they see no reason why life expectancy should not continue to rise. “The linear increase in record life expectancy for more than 165 years does not suggest a looming limit to human lifespan,” they write. “If life expectancy was approaching a limit, some deceleration of progress would probably occur. Continued progress in the longest-living populations suggests that we are not close to a limit, and further rise in life expectancy seems likely.” But with low mortality and people having fewer babies in developed countries, further population ageing is inevitable. They cite Germany as an example. Even allowing for immigration, its population in 2050, they say, “will be substantially older and smaller” than it is now.

The analysis suggests, however, that the health of the elderly is improving. Studies have rarely looked at people over 85, but improvements in their health are likely to translate into improvements also for the very elderly.

Although the number of cancers is rising as people live longer, and chronic diseases such as diabetes and arthritis are increasing, better diagnosis and treatment means that people can live good lives in spite of them. Obesity is expected to cause more health problems, but its consequences can be modified by the use of drugs.

“Traditionally, man has three major periods of life: childhood, adulthood and old age,” they write. “Old age is now evolving into two segments, a third age (young old) and a fourth age (oldest old).” Some experts have said the prospects for the fourth age are poor — “characterised by vulnerability, with little identity, psychological autonomy and personal control”. But a Danish study found that 30 per cent to 40 per cent of people today were independent between the ages of 92 and 100. A U.S. study showed that 40 per cent of 32 supercentenarians (those more than 110 years old) needed little assistance or were independent. These studies, Christensen and colleagues write, “do not accord with the prediction that the fourth age is in a vegetative state”.

Life in 2109

Thanks to anti-ageing tablets, by 2109 the centenarians born this year may have retained their relatively youthful looks and be playing cybergolf on holiday in the biodomes of the moon’s best hotels, some futurologists believe.

There are, though, much darker visions. They could find themselves living in a hot, frightening world, with the ancient wars of religion and oil long forgotten and replaced by horrific global conflicts as an increasingly large number of humans compete for diminishing stocks of drinkable water and tillable land.

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