What is the secret to living a long life?

A 

This year, the number of retired pensioners in the UK exceeded the number of under 18 for the first time in history. That's remarkable in its own right,but the real "population explosion" has been among the oldest of the old - the centenarians. In fact,this is the fastest-growing demographic trend in much of the developed world.In the UK,their numbers have increased by a factor of 60 since the early 20th century.And their ranks are set to swell even further,thanks to the aging baby-boomer generation: by 2030 there will be about a million worldwide.

B 

These trends raise social, ethical and economic dilemmas. Are medical advances artificially prolonging life with little regard for the quality of that life?Old age brings an increased risk of chronic disease,disability and dementia,and if growing numbers of elderly people become dependent on state or familial support,society faces skyrocketing costs and commitments.This is the dark cloud outside the silver lining of increasing longevity.Yet researchers who study the oldest old have made a surprising discovery that presents a less pessimistic vision of the future than many anticipate.

C 

It is becoming clear that people who break through the 90-plus barrier represent a physical elite, markedly different from the elderly who typically die younger than them.Far from gaining a longer burden of disability,their extra years are often healthy ones. They have a remarkable ability to live through,delay or entirely escape a host of diseases that kill off most of their peers. Supercentenarians - people aged 110 or over - are even better examples of aging gracefully. The average supercentenarian had freely gone about their daily life until the age of 105 or so,some five to 10 years longer even than centenarians, who are themselves the physical equivalent of people eight to 10 years their junior.

D 

One of the most comprehensive studies comes from Denmark.In 1998,Kaare Christensen at the University of Southern Denmark,in Odense,exploited the country's exemplary registries to contact every single one of the 3600 people born in 1905 who was still alive.Assessing their health over the subsequent decade,he found that the proportion of people who managed to remain independent throughout was constantly around one-third of the total: each individual risked becoming more infirm,but the unhealthiest ones passed away at earlier ages,leaving the strongest behind.In 2005,only 166 of the people in Christensen's sample were alive, but one-third of those were still entirely self-sufficient.

E 

Christensen's optimistic findings are echoed in studies all over the world. In the US,almost all of the 700-plus people recruited to the New England Centenarian Study (NECS) since it began in 1994 had lived independently until the age of 90, and 40 per cent of supercentenarians in the study could still look after themselves.In the UK,Carol Brayne at the University of Cambridge studied 958 people aged over 90 and found that only one-quarter of them were living in institutions or nursing homes.Likewise,research in China reveals that before their deaths, centenarians and nonagenarians spend fewer days ill and bedridden than younger elderly groups, though the end comes quickly when it finally comes.Of course,people can live independently without being entirely healthy,and it is true that most centenarians suffer from some sort of ailment. These range from osteoarthritis to simple loneliness.

F 

Not all of the oldest old survive by delaying illness or disability,though – many soldier through it. Jessica Evert of Ohio State University in Columbus examined the medical histories of over 400 centenarians. She found that those who achieve extreme longevity tend to fall into three categories.About 40 per cent were "delayers",who avoided chronic diseases until after the age of 80. Another 40 per cent were "survivors", who suffered from chronic diseases before the age of 80 but lived longer to tell the tale. The final 20 per cent were "escapers",who hit their century with no sign of the most common chronic diseases, including heart disease,cancer,diabetes, hypertension and stroke.Intriguingly,one-third of male centenarians were in this category, compared with only 15 per cent of women.

G 

Okinawa in Japan is the front runner.At 58 centenarians per 100,000 people (and rising), it has the world's highest proportion in this age group - more than five times the level of some developed countries.Like other hotspots,including Sardinia and Iceland, Okinawa is a relatively isolated island community. Of course, members of isolated communities or families usually share a particular environment too, but this alone cannot explain clusters of longevity. Gerontologists have found that the influence of social factors such as wealth fades as we age, while that of genes increases.By comparing 10,000 pairs of Scandinavian twins, Christensen found that genes only start exerting a strong influence on our lifespan after the age of 60. Before then, both identical and non-identical twins have largely independent odds of reaching a given age. Beyond 60, however,the odds of one twin reaching a given age are greatly increased if their co-twin has done so, especially if the twins are identical.

H 

This makes the "centenarian genome" a key resource for identifying "longevity genes", an invaluable step in understanding the physiological processes underlying long lives. Such genes have been found in abundance in other organisms – including over 70 in the worms. Unfortunately, it's a different story in humans. While many candidate genes have been suggested to affect lifespan, very few have been consistently verified in multiple populations.