ON FRIDAY, the National Bureau of Statistics announced that China's working-age population shrank last year. In the slow-moving world of demographics, that felt like a dramatic turning point: "peak toil", if you like. The mobilisation of Chinese labour over the past 35 years has shaken the world. Never before has the global economy benefited from such a large addition of human energy.

And now the additions are over. The ending came rather sooner than expected. The percentage of Chinese who are of working age started falling in 2011. But the number of working-age Chinese was expected to grow for a few more years yet. As recently as 2005, official projections suggested it would grow until the mid-2020s.

I'm not sure why demographers got it wrong. Predicting future rates of longevity and especially fertility is undeniably hard. But surely it isn't that difficult to figure out how many people aged seven today will become 15 (and thus of working age) in eight years' time. Therefore, it shouldn't be that hard to predict the near future of the working-age population. Perhaps the difficulty lies not with prediction so much as measurement. As I understand it, the yearly estimates of China's population are based on an annual national survey of about 1.5m people. Given the size of China's population, it would be easy to miscalculate the numbers by a few million here or there. Such errors could easily throw a projection out by a few years.

Also worth bearing in mind is the definition of working age. In last year's press release, working age was defined as 15-64 years old. That is a common age range used by the UN's Population Division and China's own Statistical Yearbook. But for the purposes of Friday's press conference, the NBS changed the definition, referring instead to 15-59 year olds. The number of Chinese in this age group declined by 3.45m, it reported (see chart). But the number of people aged 15-64 seems to be increasing still. It rose to 1.004 billion in 2012 (I inferred this total based on other numbers provided in the press conference).

There's nothing wrong with either age range. The 15-64 range reflects common international practice and China's own past definition. The 15-59 range is probably a better reflection of China's economic reality, where men can retire from formal jobs at 60 and women often retire five or ten years earlier. (According to the National Transfer Accounts pioneered by Ronald Lee and Andrew Mason, 60 is the age at which the average Chinese earns less than he consumes, becoming, in effect, a dependant.)

But it's interesting that the NBS chose to rejig the definition of working age for this press conference. One can only assume they chose the 15-59 age group precisely because its numbers are already declining. That allowed them to highlight a worrying demographic trend. In response to a reporter's question, Ma Jiantang, the head of the NBS, said he did not want the population figures to be lost in the sea of data.

It is almost as if China's statisticians decided to set the clock a few minutes fast to make sure China's policymakers have good time to prepare for their impending demographic duties.