Why do we think old age is a burdensome, debilitated state? Raymond Tallis says it is because we cannot conceive of life beyond children and careers.

Last week came the news that an estimated 100,000 people in Britain now in their thirties will live to be 100. In the United States, most middle-aged Americans have more living grandparents than children, and senior citizens outnumber teenagers.

More advances in longevity may be brought about by better medical treatments for diseases of old age, such as strokes and Alzheimer's. More important, the huge potential for preventive medicine is starting to be realised. The foundations of a healthy old age are laid in utero, and the healthier babies of the postwar years have enjoyed a lifetime of better nutrition, education and safety at work than any previous cohort. They will reach old age not only in greater numbers but also in better shape. In the UK, old people will have access to better housing and more leisure opportunities than their predecessors. For most old people, life has never been so good.

This optimistic tone may provoke disbelief and concern. Is it not true that the longer people live, the more chance they have of becoming a burden - to themselves, to relatives and to the economy? Don't old people cost money, time, effort, patience? Is there not evidence of this "burden": crisis in the health service with hospitals overrun by ill octogenarians; tales of carers crushed by the demands of caring; panic in the Treasury as health and social care budgets spiral?

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So here is a paradox. The debate on the implications of an ageing population is highly negative. Gloomy about the present and doomy about the future, it focuses on the pressures on the health service, the coming crisis in the economy and the need to liberalise the law relating to euthanasia. Meanwhile most old people are in good health and enjoying a quality of life their predecessors would have found unimaginable.

Stories of over-burdened hospitals are the tip to which an iceberg of the wrong size is attached. The pressure on hospitals is a reflection less of the deteriorating health of old people than of rising expectations of care (ie good health). It tells us nothing about what being old is generally like.

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Let us not overdo the optimism. Old people eventually die and, en route to death, they fall ill. But this is scarcely news. What is news is that this occurs later rather than earlier in old age. Doomsayers find it difficult to believe this: the assumption that a longer life means a longer death dies hard. This assumption is unfounded. There is no evidence in the United Kingdom that longer life is bought at the cost of a longer death.

Surely, it will be argued, this cannot be true. If you do not die at once of a disease, you will die of something else - probably worse. Those doctors who stop you perishing from a (relatively cheap and quick) heart attack are simply saving you for (a very expensive and prolonged) dementia. If we are going to have long, healthy lives and yet not live for ever, what are we going to die of? And given that we are going to die of something, will we not be ill in old age?

The answer must be, yes, we will be ill at some time in our old age; but what matters is how ill and for how long. And this is where ageing - a harmonious and largely symptom-free decline of the organism - comes to the rescue. Delaying the onset of disabling disease until intrinsic ageing has raised fatality by reducing adaptability means that the period of disability before death will be shortened. The biological argument is borne out in practice. Evidence shows that, on average, the older you die in old age the less the period of serious morbidity before death - because aged organisms can withstand less than the younger one.

A progressive improvement in the quality of old age, in which the health span of a long life gradually approximates the life span, is feasible. It will not happen spontaneously: resources will need to be invested in ensuring that the improved health of older people more than offsets their rising numbers.

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Talk of the need for more resources will place health economists on red alert. Where will these come from, especially as it is predicted that by 2040 there will be only two people of working age for every pensioner? This question answers itself: the age of retirement is socially determined. Adjustment of the age of retirement in line with average life expectancy should solve the "economic crisis" triggered by the ageing of the population.

All of this is pretty obvious, so there must be something that gets in the way of thinking straight about old age. Is doom and gloom about the increasing numbers of older people less a well-informed response to epidemiological facts than the result of prejudice against the state of being old?

For most of the population, the old, in particular the very old, are "other". Everyone is, or has been, young. We have known the condition from the inside. In contrast, no one "has been" old in the past tense. Older people are ripe targets for stereotyping. This may underpin a resentment that questions the very point of being old. If, after all, older people are not ill most of the time, perhaps they should be, as befits their station in the ante-room of extinction. A perfectly fit 95-year-old who enjoys travelling the world may awaken a certain amount of envy.

But perhaps there is a deeper basis for this - a sense of the pointlessness of "just continuing" year after year. Being old underlines the ultimate futility of existence in two ways: the old are reminders of our mortality; but they are also living on beyond the stories that drive and sustain us. Living beyond career and child care, beyond the alibi of development and of promises coming from a distant, better future, old people are forced to endure being what they are, stripped of the narrative of the unfolding CV. The trajectory of life has flattened; and this can seem like preparation for the terminal drop.

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If we are to think rationally about the demographic revolution, we must recognise these anxieties. And recognise too that survival in good health in old age offers the possibility of a new kind of life beyond the traditional, largely unchosen narratives of ambition, development and personal advancement and the biological imperatives of survival, reproduction and child-rearing.

Until we learn to think more deeply on what being old tells us about human possibility, there is the danger that a long life and a healthy old age will not seem worth striving for. This is the International Year of Older People. It may be an appropriate time to ask the shocking question: 'What is the point of being old?' The answer will turn out to be neither shocking nor gloomy but rather exciting.

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Raymond Tallis is professor of geriatric medicine, University of Manchester.