For many years, the Tokyo Metro has been waging a cross little campaign to improve the manners of an already well-mannered travelling public. Unswervingly critical of the young, intrusive posters have appeared begging ladies not to adjust their mascara on the trains (they actually deserve an award for dexterity) and others imploring men not to sprawl on the seats.

But the angriest of the bunch shows a cheerful-looking youngster enjoying some music on his headphones with a balding, tie-pinned salaryman sitting beside him fuming. Yes, the tinny “tss-tss” of headphones can be annoying, but the poster's command - “please do it at home” - is grim, unnerving stuff. What it seems to be saying is that the country that for ever changed the world with the invention of the Walkman would now prefer its people to abandon portable music altogether, especially when the rancorous older generation is anywhere nearby.

Worse still, the Tokyo Metro's irascible message comes as Japan seems to be collectively forgetting what made it great. Sure, Japan Inc still has plenty of well-funded corporate garden sheds churning out gizmos, but the eccentric streak of inventiveness is in crisis. There is a horrid feeling that a country that used to behave like a zany, experimental young firebrand and apologised to no one is becoming that fuming, timid, balding old salaryman who isn't interested in the latest gizmo because he can't see the damn buttons through his bifocals.

For Japan, technically and figuratively, is losing its youth. There are all sorts of socio-economic issues bound to spring from this poisoned well, but the immediate horror is that Japan may simply be demographically less inclined towards novelty. With a world-beating average age of 44 (versus figures in the 30s for the US and China), the country is simply older and crankier than its international rivals. Einstein's most original ideas spewed out of his noggin before he was 30.

The question - and for the future of Japan's economy it could turn out to be a trillion dollar question - is whether, when it comes to the absolutely got-to-have-it, trailblazing big idea, Japan has somehow mislaid its inventive mojo.

Oddly, national disquiet over this issue seems to have reached Japan's mostly moronic pop charts. Vapid warbling has given way to hard-biting politics: “Fight, Japanese salaryman and recuperate our dignity!” croon the pretty boys and girls of Aladdin, before unleashing the savage stanza “If you've got a brain, use it! If you've got muscles, use them! Believe me, Japan isn't licked yet!”

More significant is the vein in which the song continues: a mawkish cavalcade of the inventions that a now supposedly sagging Japan has given the world. The list, which includes instant noodles, canned coffee, karaoke and the blue light-emitting diode, serves as a reminder to Japan's youngsters of the world-class inventive legacy they must soon steward.

The problem is that in most places you look in Japan, that legacy seems to be either in outright decay or in the hands of an older generation pathologically unwilling to hand the keys over to their successors. Certainly, there are some quite brilliant anomalies in this picture: the United States and Europe have vast electronics, automobile and biotech industries, but none of them invented the Nintendo Wii, the Toyota Prius or the synthetic embryonic stem cell.

And international rivals jealously acknowledge that, as engineers, miniaturisers and filers of patents it is hard to touch Japan. But, as you walk through the electronics zoo of Akihabara, or the gizmo caves of the Tokyu Hands department stores, there is now an unfamiliar flatness to it. There is plenty of colour, and plenty of genius in the detail, but there is a marked shortage of the next big idea. You used to see the phrase everywhere in Japan, but somehow nothing is “epoch-making” any more.

Far, far too much time has been spent agonising over the question of why Sony did not invent the iPod. A casual mooch through corporate Japan shows that this is a crisis far bigger than one company: Sharp, Panasonic and Toshiba didn't invent the iPod either but are not browbeaten by the technology press in the same way. And it is silly to make a particular fetish of the iPod in this debate: Japan didn't come up with the BlackBerry, YouTube, the $2,000 car, Google or the clockwork radio.

A large part of the creativity crisis is undoubtedly economic. Behind Japan's extraordinary historical flow of new gadgets, products, and ideas has been a unique cushion of consumers ravenous for exactly those things. To a huge and under-acknowledged extent, Japanese companies never actually needed to waste time crafting the big idea - they just energetically engineered hundreds of little ones, gave them a ridiculous name, threw them into a domestic market populated with novelty-obsessed customers and waited to see which ones took off. The products that flew were, ipso facto, the lovely big ideas.

Deep-pocketed Japanese consumers were remarkably willing guinea-pigs for all this, and could be relied upon to grow bored with their new purchase within a few months. If a gadget was a flop, they didn't howl and curse on mad, partisan internet message boards: they just went and bought the next thing. The problem now is that the Japanese have stopped buying. They hold on to their gadgets for longer, they are now far pickier, value-led customers, and they are much less tolerant of a duff purchase. Tighter times and tighter pursestrings have effectively killed Japan Inc's most valuable lab rats and with them, the fount of electronic cheese.

All that, combined with the relentless reality of demographics, means that Japan badly needs to shake itself out of what could turn into a long and dozy retirement. The inventor's quest for the big idea must always tread a tightrope, with visionary brilliance on one side and fearless lunacy on the other. The older, stingier and more conservative Japan becomes, the less it will dare to walk that fine line.

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