As though to lighten the January gloom, scientists at King's College London have started the year with happy news from the world of medicine.

Exercise, the more the better, is good for you. Does that count as a breakthrough?

In a small way, perhaps it does. What the people at King's have also found is that regular exercise seems almost to defeat ageing. Once again you think: since when did that count as a discovery?

The reality is a little more startling. Working with a group of - admittedly pretty fit - amateur cyclists, the scientists struggled to spot any difference between those aged 55 and those aged 79. In other words, the traditional frailties of age are not inevitable. An individual approaching 80 can be impossible to distinguish, physiologically, from someone who is middle-aged. So get on your bike.

For many others, things are not so simple. Even if they avoid the usual, well-advertised vices, even if they eat well, sleep well and exercise, cancer can turn up. It's a price bodies pay in a society in which lifespans are extending year upon year. And that, in turn, challenges society.

Macmillan Cancer Support is clear. It concludes that, this year, 2.5 million people, "a record", will be living with a cancer diagnosis in the United Kingdom. The charity, no doubt aware that the NHS is back on every politician's agenda in an election year, argues that a crisis of "unmanageable proportions" is in store unless those in power begin to take the issue seriously.

Survival rates are higher than they have ever been: this is a good thing. Older people who would once have died live on: a long life free of illness is also a good thing. But cancer rarely ends with the relief of the all-clear, or with the news that a condition can be managed for far longer than before. Pressures on health services are - or should be - immense and growing.

Cancer Research is also blunt: "It's essential that the next government increases investment in the NHS, particularly in diagnostics and treatments, so our cancer services are fit to deal with the increasing demand of an ageing population and can ensure the best possible results for patients."

Increased resources? When A&E departments are under siege? When English hospital trusts are declaring emergencies? When waiting times targets are being missed across the UK and the Royal College of Nursing is talking about an entire system in crisis? Aside from the idiots turning up at A&E with a bit of a cough, a lot of this is due, in turn, to an ageing population.

The politics of the NHS is one thing. The last great golden goose in the public realm is being threatened increasingly, particularly in England, by the "health care providers" of the private sector and their friends in politics. Anyone who claims not to know as much is refusing to pay attention. Powerful interests, few of them British, intend to force privatisation by any means necessary.

That doesn't alter a central paradox of the modern world. Increasingly, we defeat suffering and disease. Through medicine - or just a bit of vigorous cycling - we extend life, year by year. What's more, we expect out lives to be extended with all the support we might need. We don't think much, though, about what that means, or about the wider consequences of an ageing society.

Here's more good news. Researchers at the University of Dundee and the Scottish Diabetes Research Network have conducted a study and found that life expectancy has improved for people with type 1 diabetes. The averages still fall short of the general population, but as the brother of a diabetic - and a cancer survivor - I couldn't be happier. This is what I recognise as progress. This, self-evidently, is what medicine is for.

Even amid the winter NHS crisis, you can find such progress in numerous fields, in all sorts of contests with all sorts of illnesses. Scotland is notoriously self-handicapped, but even here the simple human evidence is all around: we're living longer. The dying Scot isn't dying as early as once he did. It's a boon. But it is not a boon without price or problems.

Macmillan Cancer Support and others want politicians to get their heads around the fact and, crudely, begin to join the dots. Despite all the setbacks and difficulties, the NHS is doing a remarkable job: decent health and a measure of longevity are now almost taken for granted. Jeremy Hunt, England's Health Secretary, spent most of yesterday in a typically pathetic effort to deny that a crisis was going on, but he made a decent point. The UK's federated health service is still admired internationally.

It doesn't get everything right: how could it? For example, part of the reason for the big increase in cancer patients identified in the Macmillan analysis is that, for too long, survival rates were too low, comparatively. Now, as the four versions of the NHS begin to catch up with other countries in the developed world, the paradox asserts itself: success comes with a cost.

Do politicians want to recognise that cost? On this, I'd rather trust the word of Macmillan and Cancer Research. How do you ensure decent social care for the survivors of cancer, for example, if your only mission in life is reduce funding for local government year upon year? How do you give any sort of guarantee to those who have come through cancer when the witless Iain Duncan Smith and his Department of Work and Pensions are busy victimising those very people?

If most of us are determined to live for as many years as modern medicine will allow, Mr Duncan Smith's assault on "welfare" should be of more than passing concern. As we live longer, chances of cancer increase. Unlike the Lottery, it really could be you. The minister's department is not exactly consumed with compassion, however, so you might want to contain your applause for his present efforts.

Mr Duncan Smith and his department should be part of any effort to cope with cancer and an ageing population. What are the chances? But those of us who are not yet old and mostly fit - though God knows - need also to think about the world we are creating, the world we are demanding. Already we talk as though someone who dies at 70 has gone "too soon". Do we have an upper limit?

As the King's researchers know perfectly well, not every septuagenarian is properly fit and nifty on a bike. Some who might have once made the claim find the road ahead blocked, suddenly, by cancer. Macmillan Cancer Support is saying, very simply, that the NHS will soon struggle to cope. And cancer is far from being the only phenomenon at stake. It would take an entire second column to explore what Alzheimer's, to name one, could mean.

Older people believe, rightly, that they have paid for what the NHS can provide. You could doubt, though, that so many of the aged expected to live so long, or need so much. Those younger need to consider their own expectations.