For today's centenarians, living to be 100 is an achievement marked by a message from the Queen. Within two generations it could be as routine as collecting a bus pass.
The first person to live to 150 may already have been born, according to some scientists. Worldwide, life expectancy has more than doubled over the past 200 years and recent research suggests it has yet to reach a peak.
What will the world be like when people live long enough to see their great-grandchildren and great-great-grandchildren? Extending life by adding extra years of sickness and growing frailty holds little appeal. Increased longevity is one of the modern world's great successes, but long life without health is an empty prize. The aim is for humans to die young - as late as possible.
It is eight years since Jeanne Calment died peacefully in a nursing home in Arles, southern France in 1998. She was aged 122 years, five months and 14 days - and no one has yet challenged her title as the oldest person with an authenticated birth record to have lived. She attributed her longevity to a diet rich in olive oil, regular glasses of port and her ability to "keep smiling".
Destiny undoubtedly played a part, too. If you want to grow old, choose your parents carefully. The genetic determinants of long life are gradually being unravelled, In recent years at least 10 gene mutations have been identified that extend the lifespan of mice by up to half. The good news is that these super-geriatric mice are no more frail or sickly than their younger brethren.
In humans, several genetic variants have been linked with longevity. They include a family of genes dubbed the Sirtuins, which one Italian study found occurred more commonly in centenarian men than in the general population. Researchers at Harvard Medical School in the US, convinced they have discovered a "longevity gene", are now studying whether adding an extra copy of the gene extends the lives of mice. The long term aim is to find a way of manipulating the genes to add an extra decade or two to the human lifespan.
Other gene variants affect the production of growth hormone and insulin-like growth factor (IGF), both of which increase metabolism - organisms with higher metabolism tend to die sooner. Blocking receptors for growth hormone and IGF, so slowing metabolism, provide possible targets for anti-ageing drugs.
Also promising, but still far from yielding concrete results, are telomeres, which are present in every cell. Telomeres shorten with every cell division, like a burning fuse; when they can shorten no more, the cell dies. Inhibiting the enzyme telomerase to prevent the shortening of the telomeres in effect extends the lifespan of the cell, and, as we are comprised of millions of cells, could extend life.
Ageing cannot be reversed but it may, perhaps, be delayed. The emergence of the extremely old population has only happened in the past 50 years and is chiefly due to improvements in the health, lifestyle and environment of the elderly that started in the 1950s - how we eat and drink, where we live, what we do.
Ageing is an irresistible target for snake oil salesmen and the pharmaceutical industry. Several hundred medical compounds that can boost memory and learning ability are being investigated. Research teams are examining genes for Alzheimer's disease, mechanisms that cause cells to age and die, and brain interfaces that promise to pump new life into aged or diseased limbs. The aim here is to add life to years, as well as years to life, but ageing itself is taking over as the new target for therapeutic innovation.
One promising avenue of research is to increase the resistance of cells to the stresses caused by free radicals, unstable molecules that disrupt cellular processes. There is no evidence that the sort of anti-ageing compounds sold over the internet containing anti-oxidants that promise to tackle free radicals actually slow ageing. However, delivering antioxidant enzymes direct to the cell has been shown in mice to extend lifespan by 20 per cent - pointing the way to future research.
But the optimism comes with a warning - that the consistent increase in life expectancy we have enjoyed for the past 200 years could be about to go into reverse. Some Jeremiahs in the scientific community claim ours could be the first generation in which parents outlive their children. The greatest enemy of extending life further is growing obesity, they say. Its effects could rapidly approach and exceed those of heart disease and cancer. Calculations by US scientists suggest that life expectancy would already be up to a year longer but for obesity. As Jeannne Calment indicated, wisely if unexcitingly, on her 122nd birthday, those who live moderately live long.
Ten things you can do to help increase your life expectancy
Keeping fit is the elixir of youth. Even 30 minutes of regular gentle exercise three times per week, such as walking or swimming, can add years to your life expectancy.
Aerobic exercise preserves the heart, lungs and brain, elevates your mood, can help ward off breast and colon cancer and prevent atrophy of the muscles and bones.
Gareth Jones of the Canadian Centre for Activity and Ageing in London, found that for an over-50 who has never taken part in physical activity a brisk 30-minute walk three times a week can "basically reverse your physiological age by about 10 years." Not exercising can knock off five years.
A 1986 study at Stanford University found that death rates fell in direct proportion to the number of calories burned weekly.
Mild sunburn, a glass of wine and some low-level radiation sounds like a recipe for disaster, but many researchers believe that small doses of "stressors" can reverse the ageing process.
While this "hormeosis", is not a licence to lie on a hot beach all day swigging vodka, mild exposure to certain harmful agents can trigger the body's natural repair mechanisms. The body is tricked into producing particular DNA-repair enzymes and heat shock proteins to fix the damage that has been caused. Sometimes the body's repair mechanisms overcompensate, treating unrelated damage - "rejuvenating" as well as repairing it. Hormeosis could stretch the average healthy life span to 90.
Live in a good area
It is not only how you live, but where you live that matters - and the residents of Okinawa in Japan seem to know the secret. These Japanese islands are home to the world's largest population of centenarians.
At 103, the daily routine of resident Seiryu Toguchi included stretching exercises, a diet of whole grain rice and vegetables, gardening and playing his three-stringed instrument, the sanshin.
The clean-living Seventh Day Adventists of Utah also do pretty well, living on average eight years longer than their fellow Americans.
Worst off are those living in poor, polluted urban areas such as Glasgow, where residents of the poorest suburbs have a life expectancy of only 54. Overcrowding, dirt and noise all contribute to high blood pressure, anxiety and depression, which reduce lifespan.
Be very successful
The more rich, privileged, successful and educated you are, the longer you will live. The Whitehall Studies, 1967-77, examined the health of male civil servants between the ages of 20 and 64, and found that men in the lowest-paid positions had a mortality rate three times higher than those at the top level.
The study proved that the more important a task a person is asked to perform, the longer they are likely to live; that the person at the top with the big office, shouting orders will have a more relaxed and pleasurable existence than his frustrated underlings. And it's not only civil servants: Canadian researchers found that Oscar-winners live longer than other actors because of am increased sense of self-worth and confidence.
And if you can't manage an Oscar, then only one extra year in education could increase your life expectancy by a year and a half.
Eat the right foods
Certain foods delay the ageing process and may increase life expectancy. Green leafy vegetables such as spinach and broccoli are rich in antioxidants and beta-carotene. Diets high in fruit, vegetables, fibre and omega-3 oils, and low in fat may prevent high blood pressure and heart disease.
In their low-fat diet of fruit, vegetables and rice, the long-living people of Okinawa also consume more soy than anyone on earth, and soy is linked to low cancer rates. Eating cooked tomato daily can slash your risk of heart disease by 30 per cent, found research at Harvard.
An active mind is as important as an active body. Studies show that you can boost your immune system and delay the onset of conditions from depression to dementia by keeping your brain engaged and stimulated.
Leonard Poon, director of the University of Georgia Gerontology Center found that people who reach three figures tend to have a high level of cognition, demonstrating skill in everyday problem-solving and learning. And Marian Diamond of the University of California, Berkley, found that rodents who were given problems to solve and toys to play with, lived 50 per cent longer.
Enjoy your life
Good relationships are the key to longevity. Social contact staves off depression, stress and boosts the development of the brain and immune system.
Most research shows that people with family, friends, partners or pets, live longer than those who don't. Marriage is also a good idea if you want to meet the 100-mark, adding an average of seven years to the life of a man, and two to a woman.
Indulgence, too, can be good for you. Chocolate can enhance endorphin levels and acts as a natural antidepressant, wine contains natural anti-oxidants, and laughing is good for your immunity.
Find God - or friends
It's official: having religion pays off - and not just in the after-life.
Nearly 1,000 studies have indicated that those who go to a place of worship are healthier than their faithless counterparts - and live an average seven years longer. One in 10 of the nuns of the convent of the School Sisters of Notre Dame in Minnesota have managed to reach their 100th birthday. But atheists should not despair: experts believe that a sense of community, and of belief in something larger than yourself, are vital ingredients in a long and happy life.
Jeff Levin, author of God, Faith, and Health: Exploring the Spirituality-Healing Connection, argues that a place of worship provides a social network and a source of comfort to the ageing, ill and needy.
Reduce your calories
One hundred years of hunger is what you can look forward to if you follow the Calorie Restriction philosophy. Practitioners of CR believe that by reducing your calorie intake (by between 10 and 60 per cent) you can extend life expectancy by lowering your metabolism and the production of harmful free radicals. It sounds like torture, but there is research to suggest that it works.
One study reported that participants who ate 25 per cent less for three months had lower levels of insulin in their blood, a reduced body temperature and less DNA damage. Brian Delaney, president of the California-based Calorie Restriction Society, is aiming to live to 122, and with a diet of barely 1,800 calories per day (2,500 is the normal for men).
Get your health checked
To last a century, stay ahead of life-threatening illnesses. It is possible with regular blood tests to detect the first signs of prostate cancer, one of the commonest causes of cancer deaths in men over 85.
If you're between 60 and 69 you can have free bowel cancer screening, cervical screening for women aged 24 to 64, and mammograms for women aged 50 to 70. Figures show that 95 per cent of women who had invasive breast cancer detected by screening are alive five years later.Reuse content