On the train a few weeks ago, I sat next to a burly garment supplier. He spent the entire trip – when not yelling at his ex-wife-to-be on the phone (“you made your bed love, you lie in it”) – calling clients and telling them that their prices were going up. Those who asked questions were told that factory prices were rising in China and that they were taking the hit. End of story.

I told this to a City audience a few days later during a debate with Matt Ridley on whether optimism or pessimism is the correct approach to the future – only to hear a voice piping up from the floor telling me I knew nothing about inflation in China.

This interruption came from a young dress designer at the back. She told us that the factories she uses would bump up their prices every couple of months: these days, if you don’t take the price you are given on the spot, it goes up in 24 hours. That’s real inflation.

But the interesting bit was that the fast-moving prices have prompted her to do her sums again: she is moving her production back to Europe. It won’t cost less, but it certainly won’t cost any more. It will also allow her to keep an eye on quality – which has apparently been falling as fast as prices are rising at those Chinese factories.

For more on how fast prices are rising, look at www.shanghaiscrap.com, where you can see pictures of packs of instant noodles in a Shanghai convenience store. According to the blogger, the clerks aren’t bothering to print new price labels for the noodles as prices rise; they simply “cross them out and write in the new [substantially higher] ones”. That’s also real inflation.

This is anecdotal evidence but it suggests that, given the wage pressures building behind it, the consumer price index in China might, just might, be a tad higher than the official number of 4.6 per cent suggests.

It also chimes with news of minimum wage rises in Chinese cities: pay is about to rise 20 per cent in Beijing, 10 per cent in Shanghai and 19 per cent in Guangdong. Shanghai’s mayor says this is about “rational income distribution”. But odds are it has something to do with staff shortages (young Chinese workers are becoming more demanding) and the odd strike as well.

Either way, you could make a reasonable argument that 30 years into its 10 per cent a year GDP expansion, China is beginning to price itself out of the low-cost manufacturing market. Leave out Malaysia and Thailand, says a note from Liberum Capital, and the average worker in China is now more expensive than the average worker in any other emerging Asian economy.

One thing all this should remind us about is the power of demographics. Years of one-child policies have left the Chinese population nastily unbalanced. Today, its dependency ratio hovers around 40 per cent. That’s good – it means 60 per cent of the population is of working age. But as the only children reach working age and their parents retire, this ratio changes fast: by 2040 it will be well over 50 per cent and by 2050, over 60 per cent. If employers think young Chinese workers are stroppy now, they should wait until the poor things are trying to support two parents each, as well as their own children.

However, this shift from a nation of the youngish to a nation of the old won’t just affect China’s economy. It will, if things work out as they have in the US, affect its stock market, too.

Société Générale has a neat chart that plots the growth rate of retirees in the US against the Shiller price/earnings ratio for the US equity market. And guess what? It’s a pretty good correlation. When not many people are retiring, the stock market gets more expensive (more people are saving for retirement). When lots are retiring, it gets cheaper (people take their money out). The same goes for house prices: working people upsize, retirees downsize so the more retirees you have knocking around, the less likely it is that house prices will rise.

Anyone looking for corroboration of the argument need only look to Japan where the working age population as a percentage of the total population began to fall in the early 1990s – a time that marked the start of a 20-year grind-down in domestic asset prices.

That doesn’t bode well for US asset prices over the next couple of decades given that the working population as a percentage of the total population, having risen from 1990 through to 2008, is now set to fall until some time in the region of 2030. It bodes really badly for the Chinese stock market where the ratio of working-age people to retirees will shift much faster – starting in 2014, according to Standard Chartered. By 2030, the median age in the US is forecast to be 40. In China it will be 41. Just one more reason to stay out of the Chinese stock market.

Merryn Somerset Webb is editor-in-chief of Money Week and previously worked as a stockbroker. The views expressed in her column are personal.