Today's pensioners are enjoying healthier and longer lives than their forebears ever had but scientists - and insurers - are asking whether today's youth will reap the same longevity after a lifetime of overeating and inactivity.

The situation was spelt out yesterday at the British Association for the Advancement of Science conference in Salford by Raymond Tallis, professor of geriatric medicine at Hope Hospital in Manchester. "Nearly two thirds of the increase in longevity in the entire history of the human race has occurred since 1900," he said.

In 1901, 37 per cent of all deaths were of children under four years old, and only 12 per cent of those over 75; in 1999, just 0.8 per cent of deaths were among under-fours, while 64 per cent were aged over 75. In that time, life expectancy -- the average, not maximum, age a newly-born could expect to live -- grew from 66 to 74.5 years for men, and from 70.5 to almost 80 for women.

But will that keep happening? Professor Steve Bloom, whose group at Imperial College last week announced the discovery of a hormone that may keep appetite in check, said: "Unfortunately, in our current environment, food is freely available and the good times just keep on rolling. Society also conspires to prevent us burning off excess calories. A century ago if you wanted to go somewhere you walked, something now considered crazy."

At present, he said, there were 1,000 "premature" deaths every week in Britain from being overweight - which led him to propose a simple "tax on calories in food".

Certainly those who survived the Second World War entered an age where things could only get better. Penicillin and antibiotics had been discovered, offering a cure for diseases that had killed thousands of babies and mothers in childbirth. In the post-war years, the national diet was restricted - and later studies showed calorie-limited diets increase longevity.

"In rats it reduces the ageing process," Professor Tallis said. "And replacing post-war food with McDonald's wouldn't necessarily have been good." Those becoming pensioners now also benefited from national health care - replacing the pre-war commercial service - and good education.

The result: today's old people were much healthier than you might expect from perusing the media, Professor Tallis said. "The vast majority of old people are in good health and enjoying a quality and style of life that their predecessors (at any age) would have found unimaginable. The amount of time older people spend waiting in A&E departments on hospital trolleys is of course outrageous ... But the number of old people on hospital trolleys is a very inaccurate measure of the underlying health of the older population - after all, even the unfortunate minority spend only a tiny part of their later years in this position."

The insurance industry is also considering the implications of longevity (longer lives mean smaller payouts for those on its annuities). Actuaries, the number-crunchers for calculating the size of payouts, are not so certain we will keep living longer.

Tony Leandro, secretary of the Continuous Mortality Investigation Bureau at the Institute of Actuaries, said: "The growth in obesity may have an effect on overall longevity but it's difficult to make allowances now. Other effects such as medical advances, diet improvements, health education, less smoking, better health care and quality of living, all improve it. Then again there may be diseases that come along that we can't foresee - Aids was like that."

The bureau saw an upward blip in death rates among those in their 20s in the late 1980s, now seemingly over and reckoned to be due to Aids. "But then there's Sars and so on. We can't be sure," Mr Leandro said.

But medicine is definitely lengthening our lives. Professor Tallis said: "If you look at life expectancy gains in the past 50 years, about half is probably due to medical care, because we've had a better healthcare system in place. That hasn't changed but the medicine has."

YVONNE AND KEITH GOLDS, 67 AND 71: 'I barely knew what sweets were - and we walked everywhere'

By Oliver Duff

Yvonne and Keith Golds might be considered typical of a generation brought up on a shoestring during the war. As young children they had the run of nearby streets and fields and ate a hardy diet of meat and two veg. Sixty years later, now living in Haywards Heath, West Sussex, they both enjoy active retirements and attribute this to their sturdy upbringing.

"I think the reason I'm healthy now comes from the way life was as a child - living in the war had a huge effect on us all," says Yvonne, 67. "We ate lots of vegetables, meat and fish - and it was what I call proper food, not with chemicals or E-numbers. Because of rationing I barely knew what sweets were, we played a lot of sport at school, and we used to have complete freedom to run around the town and play. We just burnt it all off! And my parents never had a car so we walked everywhere."

She believes that regular exercise and a good diet are key to maintaining good health, and that there is a couch-potato timebomb ticking among younger generations. Where today's family of four might take their holiday by a pool in the Med, the "war generation" went on camping holidays, owned radios instead of a TV, and never fell under the "spell" of junk food.

Ms Golds plays bowls three times a week, goes to a keep-fit club and boasts that she can still touch her toes. Her husband, Keith, 71, has angina but still spends a lot of his time playing bowls, walking and gardening. The pair have recently been on walking holidays to South Africa and China, and have no plans to stop. "I'm a firm believer that the only way to keep healthy longer is to stay busy," she says.

So how does Mrs Golds think the under-50s "health bomb" can be defused? "The responsibility is the parents'. If you're brought up to eat and exercise right you don't think twice about it."

GREG ROWLAND, 35: 'I'm a child of the jam butty'

Greg Rowland admits he is not a "healthy" person. He smokes too much and eats junk food. The limits of his exercise are catching his kids in the park, and his brand consultant job consists of sitting in front of a computer all day.

He goes so far as to blame the more healthy war generation for making the Seventies "the age of the jam butty. Our parents had come off the back end of rationing," he says, "so they were more likely to give us treats."

Does Mr Rowland's knowledge of his unhealthy habits change affect his attitude at all? Not likely, he says. "A bit of what you like is good for the soul."