We are all in for a shock. It's not bad news. In fact, it's the best news possible. People are living longer. Even good news is disturbing, though, when we are unprepared. And our species is about as unprepared for the longevity revolution as the dinosaurs were for the arrival of mammals. As a result, mankind's greatest triumph - a more than doubling in life expectancy over just eight generations - might very easily turn sour.

The increase in longevity should not be a surprise; it's been going on for 200 years. Life expectancy within the UK has been increasing by two years a decade for as long as any of us can recall. That means that for each hour that passes, life expectancy increases by 12 minutes.

So why have governments not seen the longevity revolution coming? Why have they not made the gentle adjustments to our course that would steer the ships of state smoothly towards a world where we all live so much longer and enjoy better health and vitality in old age? The answer lies in a combination of short-termism and denial.

Short-termism is a problem for governments, particularly those which rely on democratic re-election. As we know from the failure to address climate change, processes that occur gradually over decades tend to be poorly addressed by governments which must account for their actions every few years.

But it is denial that underlies our inertia about ageing. None of us likes to think about ageing when we are young, and that goes for opinion-formers and politicians. By the time we are ready to address it, our influence tends to be on the wane. Such denial nourishes the ageism that pervades society. Old age is overwhelmingly seen as a time of loss - of teeth, of hair, of speed of thought, of vigour, of sexuality. But for most people, the reality of ageing can also be a process of growth, bringing satisfaction and self-knowledge.

A key element of the traditional view of ageing has been that the processes leading to frailty, disease and death were somehow programmed into us as a fixed feature of our biological make-up. It was this idea that led the UN in 1980, and again in 1990, to forecast a flattening out of the increase in longevity, forecasts that have proved spectacularly wrong.

The idea was that once the gains that had been made from controlling early mortality, mainly by improved sanitation, vaccination and antibiotics, had run their course, we should find ourselves left with the fixed, ineluctable reality of ageing. But the past 20 years have seen no slowing of the lifespan increase. This is being driven now by declining death rates among the oldest age groups of the population. We are reaching old age in better shape than ever before.

There is no genetic programme for ageing. Science has revealed that we age not because some inner clock tells us to but because the biological systems that keep us going, by repairing all the little faults in our cells and tissues, were set for an era when the chances were that you would not live much longer than 30 or 40 years ... an accident or infection would get you first. Our bodies are programmed not for death but for survival, but we live with repair systems that suited our ancestors, in their more dangerous environment, much better than they suit us today, when we have made our lives so very much safer. Eventually, the damage builds to a level that proves our undoing.

This explains why there is no fixed limit to human longevity. The current world record of 122 years and five months will be broken. It probably also explains why we are reaching old age in better shape. The kinder conditions of modern life mean that our bodies experience less damage. And we can do a lot to improve our chances of healthier ageing, through choices on lifestyle and nutrition. One of the stark injustices in today's Britain is the 10-year difference in life expectancy between the shortest and longest living regions.

Certainly there are challenges ahead as lives lengthen. Pensions, for example, need a radical rethink. Retirement at 65 was introduced a century ago when average life span was 10 years or more before retiring age. It is now about 15 years beyond it. It is small wonder then that the system is feeling the strain and we may need to work longer. But is this really so dreadful, so long as it is introduced fairly? We enjoy longer lives and better health than our grandparents, and continuing in work gives you a better income than most pensions.

The building of a society that celebrates age is not an act of charity. Combating ageism and its ills is not about 'them and us'- it's 'us and us'. You will be old too. Bend the ears of politicians now and we will all share the benefits. We owe it not only to ourselves but to the generations before us, who fought to overcome the scourge of early death so that we now enjoy the right to become old.

· Tom Kirkwood is co-director of the Institute for Ageing and Health at the University of Newcastle upon Tyne. His books include The End of Age, based on his BBC Reith Lectures.