If you're not past your sell-by date, maybe you're entering the Fourth Age

ALL would live long but none would be old said Benjamin Franklin in 1741. As the 20th Century draws to a close, life expectancy has certainly increased, but no-one likes to think of themselves as old.

And how old is old these days? Is a 45-year-old woman "past her sell- by date" as the Pennell Initiative, a pressure group set up to help older women, yesterday accused doctors of thinking? If so, how do you account for Elizabeth Buttle, who recently gave birth aged 60? When should a man retire? Warren Beatty and Robert Redford have hung on to their heart-throb status, despite being over 60. Does active life end at 65? John Glenn certainly doesn't think so. He plans to orbit the earth in October - at the age of 77. Society is marching forward, extending the age at which we do things.

This week the Debate of the Age was launched, with a commitment to ask 30 million people how they feel society should be managed in future with substantially fewer young people and substantially more older ones. The organisers of the debate have set it up knowing that demographic change in the next century is such that we have to revise all our perceptions of what we call age.

Had you been born in 1841, you could expect to live to 40 if you were a man, and 42 if you were a woman, so even 39 would have seemed old. By 1950, this had risen to 66 years and 71.5 years respectively. Male babies born in 1993 can look forward to 73.8 years of life and females 79.1.

Longevity has much to do with the fact that we are all healthier - but some are healthier than others. "The difference is in socio-economic class," says David James, professor of foeto-maternal medicine at Queen's Medical Centre, Nottingham. "You can have a 40-year-old who acts 60 and vice versa, but it depends on socio-economic background - if you're well off you're better nourished, healthier and less likely to smoke."

Many women, like the actress Patricia Hodge, are waiting until their forties before starting a family, despite the conventional "older mothers" being defined as a sprightly 35-plus. One of the biggest tolls on women's health, frequent childbearing, has almost ceased, thanks to the Pill and the decision to have children later.

"The menopause is simply a phenomenon of this century," adds Professor James. "Women simply didn't live long enough to go through it 100 years ago."

But Dr Kevin Morgan, senior lecturer in gerontology at the University of Sheffield, argues that cultural changes have been just as important as biological ones. The difference is in what we do, not what we are. "In the past we used to measure age by whether we were too old or too young to do things. It was a social construct, but an elastic measurement".

"There are so many positive role models of empowerment - such as John Glenn or Barbara Castle - that people feel there aren't just obligations but options."

Simon Knighton, director of the Debate of the Age, says: "Our attitudes need to change. There are going to be more old people and there are going to be more opportunities. The end of life debate is going to be as important as the pro-life debates of the 1960s and 1970s.

"We used to talk about the Third Age. Now we are thinking about the Fourth Age. To say what is old age is a meaningless statement." Or as the American statesman Bernard Baruch put it: Old age is always 15 years older than you are.