There is a kind of uber-story about the future of ageing, regularly featured in the press, that swings through from the lows of crisis reportage (we’ll all be living longer, the pension pots will be empty, health services will crumple under new obligations to the elderly) to the highs of techno-euphoria (gene therapy and nanotech will enrich our late lives, we’ll be able to see our grandchildren’s children, work until we’re 90 and dodge decrepitude).The story is rooted in a collective fantasy about tinkering with human longevity that speaks far more profoundly to our fear of dying than to how we might actually live our lives as older people. The trouble with this neo-Malthusian story of octogenarian overcrowding is that it doesn’t accord with what we know – which is that the human lifespan has proved stubbornly resistant to our attempts to extend it, remaining more or less unchanged for millennia. It is a species-constant given, subject only to minor variations of the cultural and environmental kind. Today, even in those developed countries awash with shiny new technology and wealth, we still can’t expect to live much beyond the ancient Pslamist’s ‘three score years and ten’, and in poor countries mortality figures skew lower still. Granted, there are more people now living into their 90s – the privileged few among the prosperous nations, but these statistical outliers ought not to cloud the big picture view.

In any case, if I look around the ageing members of my own family, with eyes stripped of fantastical visions, I do not see much to recommend longevity as a goal in itself. One of my near-octogenarian aunts, only just recovered from multiple bypass surgery, is now battling spinal cancer. She has been bedridden for most of this year, her body addled by powerful drugs. Another ageing relative is confused but bloody-minded, incontinent and, though he’d be hard pressed to admit it, resolutely bored. My own mother is practically deaf, painfully slow, frustrated by the growing limitations and daily depredations that have left her fearful of the world.

This is the kind of ageing – dogged by loss and hazard – that we long to escape, even though we persist in modelling it for ourselves over and again, changing the metaphors, but not the underlying scenarios of inevitable decline. We plot our lifespans along Bell curves that soar, and then fall. Or we bud, bloom, ripen, and rot. We sigh over Shakespeare’s seven ages of man – a sorry progression, when all we can look forward to are wheezing chests and voices turned back towards “childish treble”. And this, only as a prelude to oblivion: “sans teeth, sans eyes, sans taste, sans everything”. In the no-nonsense language of the machine age, we face breakdown, redundancy, obsolescence.

If we want to change the troublesome paradigm (which is one way of manipulating fate) it is necessary to play with these end-game scenarios, upend and subvert them. What would happen, for example, if we messed around with the standard Bell curve? If we allow that hip replacements, cataract operations, pacemakers, arterial stents, blood-pressure medications, bolder cancer treatments, and assorted other add-ons, plugs-ins, implants or cures have materially improved peoples’ experiences of getting older – lengthening what gerontology specialist Michael R. Rose, at the University of California, Irvine, calls the “Healthspan” – than why can’t we simply redraw life’s arc? We could swap the steady decline that begins in middle age for an attenuated plateau, say. On this model, seniors might enjoy an extended period of good health that opens up rather than shuts downs options for living.

Alternatively, we could flip the Bell curve like a skipping rope so that it forms a U-bend, dipping in the middle before rising again. This curve correlates nicely with findings by psychologists and psychiatrists that suggest that people become increasingly happy as they age. Most such studies rely on self-reporting among the survey group, but the results have been so reliably replicated the world over that it looks as if the happiness dividend, payable in old age, runs very deep in humans, as though it were part of evolution’s bargain with decrepitude. So while advancing age might strip us of lustre, vigour, and cognitive acuity, we gain things too: our minds might be less sharp, but they excel at making connections and evaluating choices. We experience less stress, in consequence of having a better perspective on things, and as we age we exert better judgement.

It is middle age not old age that represents the greater challenge, since in cultures as diverse as Maori, Scandinavian, and Japanese, it is at this time of life that people experience seismic ructions in their sense of identity and purpose. They feel lost, mournful, undone. The results of those happiness studies findings suggest that once you survive your midlife crisis you can soar up again like a rocket. Perhaps this phenomenon is what captured Margaret Mead’s attention, when she noted how in the years following menopause, women seem to experience a surfeit of energy. She called it “zestfulness”. I like the idea that old age brings with it a new zest for life. And for me, at least, it’s not about wearing purple, drinking brandy, and thumbing your nose at a society that pays you no heed, as the famous poem would have it – it’s about re-engaging with the world with new focus and acknowledged maturity.

I recently have read Diana Athill's new book Alive, Alive Oh!. At 99, Athill is a poised picture of sagacity and grace. Bright-eyed, her skin aglow with vitality and with a mind sharpened on a regimen of reading, writing, and reflecting, she coyly claimed that she hadn’t really planned to write another book in her nineties. Yet when chided by her publicist, who said, “Why not give it a go?”, she thought, “Why not indeed.” I mention this anecdote because it strikes me as a clear example of purposefulness, and I’ve learned from reading Atul Gawande’s cleareyed and humane book Being Mortal, that having purpose is the key to finding fulfilment and meaning in old age. Gawande, a practising surgeon, eloquently makes the case that as we age it is less medicine that we need than a life as rich in activity and meaning as circumstances will allow. It is the very opposite view of ageing to the one that has dominated Western cultures for decades, and which hinges on the idea of retirement – as though all old age were good for is taking cruises and playing golf. Now we know better. We know that retirement can lead to dreadful feelings of isolation and depression among the elderly. That boredom is corrosive, and idleness disabling.

We may not know much about the future of ageing in a crystal ball gazing sense, but we know that the Baby Boomers got it wrong. Old age isn’t a state of entitled abnegation from society, where you rest on your laurels, and live off the productivity of the young. To age happily and healthily we need to bring to bear on our futures everything we’ve learned in life. We need to exercise our bodies and minds, to keep abreast of the concerns that preoccupy society, especially its youth. As the psychoanalyst Erik Erikson argued, we need to give back to others the benefits of our long experience, becoming “a numinous model in the next generation’s eyes”.

But perhaps the best suggestion for a psychically healthy old age was Jung’s idea that if the sun is going down on us as we age, then our challenge is to trap those diminishing rays and turn them in on ourselves in order to illuminate the workings of our inner minds. Self-understanding is the best future we can hope for as the years pile up on our shoulders, representing not a burden, but a kind of a decorative honour.