With the number of "super-aged" societies set to quadruple by 2020, the smaller workforces and lower household savings that come with gray hair are likely to weigh heavily on global economic growth, analysts said.
"The demographic dividend that drove economic growth in the past will turn into a demographic tax that will ultimately slow this growth for most countries worldwide," ratings service Moody's said in a note Wednesday, citing the unprecedented speed of the population's silvering.
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By next year, more than 60 percent of the 112 countries it rates will be considered "aging," with at least 7 percent of the population aged 65 or over.
By 2020, the number of super-aged societies, or those with more than 20 percent of the population aged 65 or over, will more than quadruple to 13 and by 2030, that number will surge to 34, compared with just three countries currently, Moody's said.
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"All countries, except a handful in Africa, will face either a slower-growing or declining working-age population, and corresponding pressures on labor supply" through 2030, it said, noting 16 countries will see their working-age population fall by more than 10 percent over the period. "Population aging will also reduce household savings rates, which will reduce investment."
It cited Conference Board data indicating aging will slow aggregate annual growth rates by 0.4 percentage point in 2014-19 and 0.9 percentage point during 2020-25, shaving the 2.9 percent average annual growth rate over 1990-2005.
It's not just a developed markets problem, with many emerging markets aging even more rapidly than their developed peers.
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China's working age population is expected to fall by 2.7 percent over 2015-30, while countries as diverse as Cuba, Russia, Hong Kong and Croatia are expected to see their working-age population fall by more than 10 percent over the same period, Moody's said.
The aging population is "Asia's foremost long-term headwind," HSBC said in a note Thursday.
It's not just Japan, where sales of diapers for the elderly are already outstripping babies' diapers, HSBC said.
"The rest of the region is quickly catching up, with other East Asian 'tigers' (South Korea, Hong Kong, Taiwan and Singapore) expected to age at rates even quicker than that of Japan," HSBC said.
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"While the Western world had abundant time between industrializing and the start of the ageing process, Asia has less time to adapt," HSBC noted, adding that welfare systems haven't had as much time to develop effectively to support the grayer populations.
It estimates emerging Asia could see an average 0.6 percentage point a year shaved off its trend economic growth over 2014-30.
Asia also faces a "double-edged demographic dilemma," with the problems of a declining workforce compounded by the countries with the lowest fertility rates also having low workforce participation rates by women, HSBC noted.
—By CNBC.Com's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1