WHEN Judy Meade turned 50 she bought herself a speedboat and learnt to water-ski. That was 50 years ago.

This year she marked her 100th birthday with a regular glass of red wine and a few choice remarks about those who feel sorry for the aged.

“I haven’t seen a doctor in years,” said the Atlanta-based widow. “I don’t believe in them. They haven’t done a darn thing for me. There really isn’t anything difficult about being 100.”

As a centenarian in rude health, Meade is far from being unusual. America is beginning to wake up to a demographic revolution that is forcing officials to redefine old age and life expectancy.

As the 20th century’s postwar baby-boom generation edges towards retirement, scientists are predicting explosive growth in the segment of the population aged 100 or older. There are currently more than 80,000 centenarians in America; by 2050 there will be more than 1m.

Despite a constant drumbeat of warnings that everything we eat is contributing to our imminent demise, recent studies have confirmed that human longevity is steadily expanding, especially in the country that invented the fat-clogged cheeseburger.

“The centenarians we are studying disprove the perception that ‘the older you get, the sicker you get’,” said Dr Thomas Perls of the Boston University School of Medicine, which is conducting the world’s largest research project into the health and lives of 100-year-olds. He added: “Centenarians teach us that the older you get, the healthier you’ve been.”

In a study of 800 centenarians and around 700 of their siblings and children, Perls and his team found that neither education nor socio-economic status played a significant role in living to 100.

“Some have no years of school, others have postgraduate degrees. They are very poor to very rich. They are strictly vegetarian or have diets extremely rich in saturated fats,” he said.

Yet several important patterns have emerged. Men who live to 100 tend to have lower than average blood pressure and are almost always lean. Most centenarians never smoked, and those who did gave up on average in their late fifties. But a few, like Meade in Atlanta, still smoke up to two packs a day.

The Boston study also found that women who give birth naturally after the age of 40 have a four times greater chance of living to 100 than other women. Bearing a child in your 40s may be a sign that the body is ageing slowly, Perls said.

Dementia is not inevitable. The study found that 30% of centenarians had no significant changes in their thinking abilities. A separate study of 100 centenarians by a nursing home chain found that 27 liked to watch the MTV music channel, 15 played video games, and four knew how to use an iPod.

The last US Census revealed that there are already 1m Americans aged 90-94. Improvements in health care will enable many of those to reach three figures, but there is one statistical imbal-ance that science has not yet been able to correct.

At the age of 85, there are only 46 men for every 100 women. Centenarian romance has been found to be rare.

Times Recommends

• Tehran cannot live up to its rhetoric
• The new unicycle that will ‘change the world’
• We cannot wish away Iran’s bomb