BEIJING -- China's demographic timebomb is ticking much louder with the first fall in its labor pool for decades, analysts say, highlighting the risk that the country grows old before it grows rich.

The abundant supply of cheap workers in the world's most populous nation has created unprecedented cost efficiencies that underpinned its blistering economic expansion over the past 35 years, propelling the global economy forward.

But now the inexorable consequences of the one-child policy imposed in the late 1970s are beginning to appear, and threaten to impact its future growth.

China's working-age population, defined as 15-59, fell 3.45 million last year, official data showed earlier this month -- the first decline since 1963, after tens of millions died in a famine caused by the Great Leap Forward.

The immediate effect may be small in a nation of 1.35 billion people, but the cumulative effects will accelerate over the coming decades.

The number of people aged between 15 and 64 will drop by around 40 million between 2014 and 2030, said Wang Guangzhou, a researcher with the Chinese Academy of Social Sciences (CASS), a government think-tank -- more than Poland's entire population.

"The population is aging so fast that we are running short of time to deal with it," said Li Jun, also of CASS, adding the family planning policy had exacerbated the problem.

China's proportion of over-65-year-olds is projected to double from seven to 14% over only 26 years -- a key demographic measure that took the United States 69 years to complete.

"Undoubtedly it will substantially slow down China's potential growth rate," said Yao Wei, an economist with Societe Generale in Hong Kong.

An ageing population not only means fewer people available to employ and higher labor costs, but investment -- a key driver of China's growth -- will be harder to maintain as families spend their savings on health care, she said.