Am I going to make my century? It is a question more and more of us ask ourselves as 70 becomes the new 60, 80 the new 90 and 90 the new 80. The odds are still against getting to 100, but they are shortening all the time: last week, the Office for National Statistics predicted that a third of all babies born in the past year are likely still to be alive in 2112 - and the number of centenarians will rise to almost half a million by 2050.
Science, meanwhile, is coming up with some crucial answers to the all-important question: what is the secret of reaching the magic number?
A healthy diet and lifestyle are clearly key, but research at the University of Boston, published earlier this year in the science journal PLoS One, suggests that genetic factors may be more important. While our genes are only thought to influence our chance of living to 85 by about 20 to 30 per cent, the Boston study suggests that they could play a far greater role in our survival chances into the late eighties and beyond.
When researchers scanned the genomes of 800 centenarians and compared them with similar samples from a control group, they identified 281 genetic variants that appeared to play a role in ageing. These were grouped into 26 different ''genetic signatures'', shared by 90 per cent of the centenarians in the trial. (The genome is someone's entire hereditary information, encoded in their DNA.)
In other words, while going to the gym and staying off cheeseburgers is all very well, your best chance of reaching 100 could simply be having a centenarian for a parent or grandparent.
Although there is no single ''ageing gene'', the genetic signatures revealed by the Boston study could yield "a better understanding of the genetic basis of delaying or escaping age?related diseases", says Dr Thomas Perls, a lead author of the report and director of the New England Centenarian Study.
Whatever conclusions the scientists come to, and wherever further research leads, the elders of the tribe, living decades longer than their contemporaries, will always be an object of veneration and fascination. Every centenarian has a different story to tell, with some seemingly reaping the rewards of living a healthy life while others defy the years despite having, in some respects, outrageously unhealthy lifestyles.
As this cavalcade of centenarians past and present shows, there are almost as many ways to live to 100 as there are to die young.
Lovers of life
Wisecracking comedian Bob Hope reached the 100 mark in 2003, and died a few weeks later. "I'm so old, they cancelled my blood type," he once joked. The British-born star was as physically active as he was quick-tongued. A voracious womaniser, Hope was also a fanatical golfer, playing well into his nineties. Whatever his secret, it was clearly infectious. His wife Dolores lived to 102.
The late Queen Elizabeth, who lived to 101, cheerfully broke the medical rules, consuming, according to some estimates, 70 units of alcohol a week - five times the recommended number for a woman. Her favourite tipples included gin, Dubonnet, claret and champagne. She was famous for her sense of mischief, led a busy social life, and relaxed by reading PG Wodehouse novels.
The oldest centenarian whose birth date has been reliably documented was a Frenchwoman, Jeanne Calment, who died in 1997 at the age of 122 (and therefore qualifies as one of the world's few supercentenarians - people who have lived beyond the age of 110). She attributed her own longevity to the copious amounts of olive oil she poured on her food. She kept active and cycled into her nineties, but also had her fair share of vices. She drank port, consumed a kilogram of chocolate a week, and smoked, in moderation, until she was 117.
One centenarian who gleefully confounded medical opinion was the American comedian George Burns, who died in 1996, shortly after his 100th birthday. Burns smoked between 10 and 15 of his trademark cigars a day, often accompanied by a martini. He dedicated one of his books to "the widows of my last six doctors".
Creatures of habit
Christian Mortensen, who died in California in 1998 aged 115, summed up the secrets of his longevity as "friends, a good cigar, drinking lots of good water, no alcohol, staying positive and lots of singing". A former milkman and factory worker, he had a mainly vegetarian diet and boiled all his water.
The Japanese centenarian Tane Ikai had a diet of stultifying monotony that would have tried the patience of a saint. On a typical day, she ate three meals of rice porridge. But it did the trick. Ikai pegged out in 1995, at the age of 116.
Great-great-grandmother Ada Marley, from Oxfordshire, who turned 100 in January, combines an abstemious lifestyle with an active mind: "I have never smoked or drunk, but I have kept myself busy with things like needlework and embroidery." She keeps herself mentally alert by doing a crossword or word puzzle every day.
Minnesota railway clerk Walter Breuning, who died in 2011 at the age of 114, practised callisthenics (a type of exercise to increase body strength) daily almost until his death. His diet consisted of a large breakfast followed by smaller meals later in the day. In old age, he skipped his evening meal altogether and ate fruit instead.
The German film-maker Leni Riefenstahl, notorious for her Nazi propaganda films in the Thirties, was also renowned for her stamina and physical resilience. She survived a helicopter crash at the age of 100, and was still a keen scuba-diver when she died in 2003, at the age of 101.
One of the last survivors of the First World War, Henry Allingham was briefly the oldest man in the world before his death in 2009, at the age of 113. As well as witnessing the horrors of war, he suffered two nervous breakdowns, but he remained upbeat. He jokingly attributed his longevity to "cigarettes, whisky and wild, wild women".
Another man to weather an inauspicious start in life was the songwriter Irving Berlin, whose family fled Russia during the pogroms of the late 19th century. The composer of White Christmas was a workaholic, often making do with very little sleep, and was prone to bouts of depression; but he passed the century mark in 1988, shortly before his death.
Genetics will play its part, but studies of people living in longevity hotspots such as Sardinia and Okinawa in Japan suggest that lifestyle is also significant.
In high-longevity societies, locals work until older than average, with more emphasis on activities such as walking and gardening.
Eat less; mainly plants
The traditional diet in Okinawa is high in vegetables and low in calories, meat and dairy food.
Get enough - but not too much - sleep
One study of more than 21,000 twins found sleeping for fewer than seven hours but more than eight hours a night was associated with a risk of dying earlier.
Centenarians have strong ties to family and friends, according to Dr Perls. They are also more extrovert and have high self-esteem.
Prof John W Santrock from the University of Texas, an author on lifespan development, found that a sense of spirituality can help ease stress.
Several studies have found that attributes such as thoroughness, reliability and attention to detail are linked to a longer life.
Limit alcohol and avoid smoking and snacking between meals. One large study from California found that these practices are linked to longevity.
Stay single - or get married
Research has also found that a disproportionate number of centenarians are women who have never been married - although other research indicates that marriage is associated with living longer.
Another subgroup which seems to have a better than average chance of living to 100, according to Prof Santrock, are those who have survived traumatic events and developed mechanisms for coping with stress.