Picture an elderly person. Who sprang into your mind? Or should I say, whose Zimmer frame hobbled slowly round the corner of your mind?

Perhaps it was Thora Hird, the hilarious comic actress. Or Prince Philip, the hilarious comic Duke of Edinburgh.

Maybe you pictured the beautiful, crinkly face of Nelson Mandela. Or was it your own dear grandma?

Whoever you imagined, I'm prepared to bet that it wasn't Nicole Kidman. When her photograph appeared last week next to the headline "Brain Games More Harm Than Good For Elderly", she must have wondered whom to sue first: the newspaper or her dermatologist. (I am not suggesting that Nicole Kidman has had Botox. No doubt her skin retains the magical texture of a Formica kitchen surface due to her being well-rested and drinking plenty of water.)

Poor Nicole was selected to be in the photographic line of fire because she is among those who have advertised "brain-training" machines on television. Brain-training machines which, according to research from American health organisation Lifespan, do not actually train the brain to do anything very much at all, except use the machine. The study, based on trials undertaken since 1992, found no evidence that these exercises "delay or slow progression of cognitive changes in healthy elderly".

Indeed, according to Professor Peter Snyder who led the research, computer games may actually be bad for our silver-haired friends. They may, warns Professor Snyder, "decrease participation in more proven effective lifestyle interventions, like exercise".

In other words: old people should stop hunching over their Nintendos and get out there in the fresh air. Play in the park! Climb trees! Do your homework! Bring back national service!

A ghastly image springs to mind: thousands of silver-haired obsessives poring over hand-held computer screens in darkened rooms, chainsmoking, stuffing cheesy Wotsits into their mouths. Their children stand on the threshold, tutting and shaking their heads. The old folk look up, roll their eyes sarcastically, flick a V sign, slam the door, turn the music up. Perry Como booms through the house, all bass and no treble. The children sigh, shrug, ring up friends to say: "I don't know what to do ... it's just tap-tap-tap all day ... I can't get him to read a book or eat an apple ... his eyes are turning square ... I hope it's just a phase he's going through ..."

All very healthy. It's about time we started disapproving of pensioners as a generation. Why should they get off lightly? We complain that children have no manners, teenagers are violent, twentysomethings are slags, thirtysomethings are greedy and the middle aged have affairs because they've lost respect for the institution of marriage. Meanwhile, for the 70s and over, it's all: "Are they warm enough? Can they live on their pensions? Should we visit more?"

Old folk have had it too good for too long. If young Billy gets a hard time for spending all day tweedling computer buttons in a darkened room, why shouldn't Grandma? Get out there in the sunshine, Gran, and stop wasting the best part of the day!

We feel, instinctively, that the elderly should be allowed to do whatever they like. Slump on the couch, eat fried breakfasts, take the best seat on the bus, wear purple; the clock is ticking, so they should aim for sheer pleasure with our blessing. How dare some American professor tell them to be sensible!

My mother has an idea for government policy that is undeniably brilliant: drugs such as marijuana, ecstasy, heroin and cocaine should be legalised, then given free to the over-65s. If pensioners are off their heads on smack, it would be no drain on the national work force. If the drugs make them aggressive, if you catch them breaking into your house at night to steal the DVD player, it wouldn't be very scary. You'd just trap them by putting a packet of Werther's Originals on a table with a net over it.

Thus, getting high would lose its cachet among the young. And old people would be relieved of their aches and pains while having a whale of a time.

It's an appealing thought, because we imagine that one of the comforts of old age is the abrogation of responsibility. Working for a living, bringing up children, getting an early night, saving for a house, eating and drinking sensibly for a long if boring future: all these worries should be behind us once we pick up the bus pass.

Nevertheless, until we are old, we don't know what it's like. If the majority of elderly people didn't want to feel young again, it would not have been so easy for computer-game manufacturers to sucker them with advertising that claims their product will "keep your brain sharp".

One of the commercials explains that the brain-training machines are "inspired by the research of Dr Kawashima". What a brilliantly opaque, advertising-style sentence. The elderly target's subliminal mind inhales "research" and "doctor", so she comes away thinking this is a development akin to the discovery of penicillin. But if you actually think about it, the words "inspired by" mean almost nothing at all. I could perform a contemporary dance inspired by the research of Dr Kawashima and that wouldn't stave off Alzheimer's either.

Then there's the fact that we don't really know who Dr Kawashima is. They might as well have said Dr Hook. Or Dr Prodd from Carry On Matron.

It works, presumably, because we are all seduced by the idea of being rejuvenated. Sod the right to indulge ourselves in hazy, ageing comfort - we just want to be young.

If the machines don't work, the answer is surely to launch a storm of disapproval at our grandparents' sedentary lifestyle. We must grumble at their love of computer sudoku and demand that they get off their lazy backsides at once. If we scold them enough, they will feel like kids again after all.

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