A remarkable transformation is underway across the world. Falling fertility rates and rising life expectancy mean there are fewer young people and more older ones. The result is an ageing society, characterised by a rising average age and a growing proportion of older people. This is often seen as a bad thing: most old people don’t work, they need a pension, and their health care is costly. The result is rising government debt and an economy staggering under the burden of a high old-age dependency ratio. But this negative approach is a strange way to frame one of the twentieth century’s greatest achievements.

When I was born in 1965, the most common age of death in the UK was children in the first year of life. Today it is 86. We are, on average, living longer and in better health for longer. How have we turned all this into a bad news story?

Most old people don’t work, they need a pension, and their health care is costly

Rather than fall prey to this pessimism about an ageing society it is about time we engaged in the same discussions about adaption and adjustment that characterise debate around Artificial Intelligence and climate change. That requires less focus on there being more older people and more on how we adapt to increases in our longevity.

For the first time ever, the young and middle-aged can now expect to become old. There have, of course, always been old people, and old people were always young once. But becoming very old was previously an experience for a minority. But now, when in the UK fifty per cent of newborns are expected to live beyond 90, it is the future for the majority.

This means that, whatever your age, you need to take steps now to maximise your chances of ageing well. We have to focus less on chronological age, defined by the passing of the years, and more on prospective age, how many more years you can expect to live. It is this forward-looking perspective that demands we behave differently and marks me as different at 58 than my father and grandfather were at the same age. An ageing society begins when you have lots of people aged over 65. A longevity society starts when you have lots who will live beyond 65.

Also important is ‘biological age’. As we live longer, the disease burden is increasingly ageing-related – cancer, cardiovascular disease, diabetes, dementia, arthritis, etc. If we can slow down biological ageing we can postpone all of these, leading to substantial gains in the quantity and quality of life. Prospective and biological age force us to recognise that ageing is malleable in a way that chronological age isn’t.

Ensuring healthspan extends to match lifespan is top of the longevity agenda. Jonathan Swift, the 18th century satirist, summarised our fear of an ageing society by noting that ‘every man desires to live long, but no man wishes to be old’. Right now, our health system turns out outcomes Swift would recognise from the Strudlbruggs he describes in Gulliver’s Travels. The Struldbruggs were immortal but aged normally and went through life dreading what was to come. That is a good metaphor for our current health system that has done a great job of slowing down dying but not ageing. We have designed a health system to treat illness but not one which keeps us healthy.

Longer lives demand a shift towards prevention and keeping us healthy for longer rather than intervening when we become ill. Genetic screening, personalised treatments and extensive use of big data are all key. It also requires greater responsibility on you as the patient. In the current system, the patient’s role is to turn up, be tested, diagnosed and treated. As people live for longer, they will have to take a far more active role monitoring your health.

If we aren’t to run out of money we also need to be productive for longer. Employment rates peak at age 50 and then decline. For the majority this is due to ill health, caring, outdated skills or ageism in the workplace. As a result, raising the state pension age will have relatively little impact. We need instead policies that maintain our health and skills for longer rather than just incremental increases in the age at which we claim a state pension. We need to recognise that when three quarters of employment growth in high income countries is driven by workers aged over 50, older workers are key for economic growth.

A longevity society also requires fundamental cultural change. In the twentieth century, the increasing medicalisation of old age defined it in terms of decline. The result is we underestimate the capacity of older people and our own later years. We need instead to recognise that longer lives open up the possibility for time spent in adult development and that ageing is not an event that happens at 65 but a continual process that begins from our early years. Only then can we move away from underestimating the capacity of older people and underinvesting in our own future and producing the very ageing society outcomes we fear – of outliving our health, wealth and purpose.

When global life expectancy exceeds 70 we have to age in an evergreen manner if we are to avoid the palsied dystopia of an ageing society. If successful we will be setting out on a dramatic new path. When we are ill in our 90s, few want to live to be a hundred. But the more progress we make in ageing better the longer we will want to live for. Humanity is embarking on a radical new era focused on changing how we age. Just don’t call it an ageing society.