Once upon a time, the consolation for reaching middle age was being allowed to go cheerfully to seed. What bliss, in a way, to finally let yourself go; throw away the hair dye, buy a bigger size, stop feeling bad about never doing any exercise, and gratefully give up the pretence of keeping up with fashion or music. Just throw on a cardigan, and let it all hang out. Liberation indeed.

But times have changed, and the era newly rebranded as “midlife” – so much less frumpy-sounding – is no longer such a comfortable place to hide. Welcome to the world of the new old, where the over-50s are almost as heavily bombarded as their teenage children by advice on staying leaner, meaner and hotter for longer.

Mimi Spencer, author of the infamous 5:2 diet book – the one that had everyone from George Osborne to Benedict Cumberbatch fasting for two days a week – is back as co-author of a new recipe book for the over-50s, which seems to boil down to not many calories plus a lot of “anti-ageing” turmeric and flaxseeds.

The point of her Midlife Kitchen regime is, she insists, not so much traditional weight loss as defying the slowing down, spreading out and softening up that traditionally comes with ageing bodies. And it’s testament to her unerring eye for a trend that you can barely open a women’s magazine or Saturday supplement without stumbling across some faintly exhausting new strategy for defying the inevitable – all illustrated by photos of what look suspiciously like twentysomething bodies, with abs so taut you could trampoline on them, spiced with anti-ageing tips of occasionally dubious scientific provenance.

Eat more walnuts! Lift weights! Have more sex, learn a new language, and don’t forget to do sudoku to keep your brain active! The self-improvement industry is moving on from a hundred different ways of saying “don’t get fat” to something more like “don’t get old”, or at any rate, not in a burdensome way.

For it is starting to feel almost antisocial, with the NHS visibly creaking at the seams, not to fight the dying of the light. Once we might have seen the painful, chronic diseases of old age as unavoidable twists of fate or genetics. But the increasing evidence that even conditions like dementia – let alone diabetes, cancer or heart disease – are at least partly linked to diet and exercise brings a creeping sense of responsibility. Striving to stay young was once considered vain. At this rate it may begin to look more like saving for a pension: a prudent investment in a comfortable old age (especially when we’ll all need to work for longer), and practically a moral duty to those who would otherwise have to look after you.

This week, a study was published suggesting that having a “brain age” – essentially a measure of how well your grey cells are holding up – older than your chronological age may indicate that you are at risk of dying young. That opens up the possibility of a test that may in future be available to every family GP, allowing them to predict reasonably confidently which of their middle-aged patients are risking serious illness if they don’t clean up their act.

It sounds unethical for doctors not to share knowledge that could, in theory at least, prevent untimely deaths, but the issue is less simple than it looks – not least because so many of those most at risk won’t necessarily want to know. I felt faintly ridiculous traipsing to the surgery for my over-40s MOT – a free NHS checkup now offered to anyone in that age bracket, to pick up early warning signs of conditions such as high blood pressure – when there’s nothing wrong with me. The nurse admitted that most of those taking the test turn out to be revoltingly healthy. The smokers, the obese and the heavy drinkers are often reluctant to take up the offer for fear of being told something awful.

So instead she sees more than her fair share of “worried wells” and men in Lycra, desperate to get a gold star for cycling 200 miles every weekend.

The more easily identifiable the causes of premature ageing and illness, meanwhile, the greater the risk of ugly public blame games. Today’s pensioners can hardly be held responsible for behaviour they never knew was so risky, but in years to come could the smug superfood-munchers – who are statistically more likely to be wealthy – start to resent paying for the consequences of what they see as other people’s failure to eat enough avocado and flaxseeds? Willingness to pool risk through the NHS – which relies on everyone chipping in through taxation because none of us knows when we might need its services – might start to look very fragile.

It’s scary stuff, which is precisely why most of us prefer not to think about it. Easier to make a vague mental note to take up jogging one day, and then cheerfully forget all about it. But human understanding of the ageing process is changing, and that in turn will inevitably change what it means to be middle-aged. Like free university education, embracing the elasticated waist without guilt may be one of those pleasures that die with the baby boomer generation.

And really, we should be grateful for it. Knowledge is power, and it’s surely better to have more control over our fates than we thought; to know there’s something we can do to stave off the dreaded prospect of decrepitude – even if it’s easier for some to achieve than others, and even if it’s not guaranteed to work.

If the prospect of lifelong devotion to soya beans and the treadmill is less than thrilling, the prospect of brittle bones, diabetes or an impoverished early retirement on health grounds is hardly hedonistic. And who would deny the over-50s a right to their own sexuality, to vitality, to not being put patronisingly out to grass?

But beware the point where the self-improvement industry realises that feelings of inadequacy and body shame no longer have an age limit, and that there’s gold in them wrinkly old hills. There’s a fine line between being thrilled at the way someone like Helen Mirren is breaking down professional barriers for older women, and being made to feel bad for not looking like Helen Mirren did at 62 in a bikini; between rightly wanting to stay fit, and feeling under wholly unrealistic pressure to stay hot. Let’s hope age still brings the wisdom to know the difference.