On the developed world's cohorts of future centenarians

When blogger James Nicoll linked to this Guardian report suggesting, after The Lancet, that "50% of Britons born now will see their century," he did so using the link label: "Experts warn of nightmarish future filled with healthy and productive old people."

Sarah Boseley's article reports that the researches of Kaare Christensen et al., recently published in The Lancet, suggest the majority of children born in economically developed nations are likely to live at least to the age of 100.

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 UK 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 UK 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 UK.

The growth in life expectancy, Christensen argues, was driven until the 1920s by declines in infant and maternal mortality, then by continuing and sustained improvements in old-age mortality.

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."

At this point, it's legitimate to recall the Greek myth of Tithonus, a Trojan prince whose lover Eos asked Zeus on his behalf that he be given the gift of immortality, but forgot to ask for eternal youth as well. There's no reason to think that poor health and social isolation must necessarily be the case for this new class of elderly, however.

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.

Christensen has some suggestions of his own, and looks forward to new research that might isolate the various genetic and environmental factors behind extended lifespans. Population aging will cause problems, even if people who live very long remain fairly productive throughout their final years, but whatever helps.