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While most of Southeast Asia is expected to enjoy relatively young populations in the decades ahead, one country appears to be bucking the trend in a big way. Thailand, the region's second-biggest economy after Indonesia, is aging rapidly and economists say not enough is being done to prepare the country for the demographic change taking place. (Read more: The aging in Asia face a new decade like no other) The United Nations (U.N.) expects Thailand's working-age population, those aged between 15 and 64, to peak in 2017. Its data show that 8.9 percent of the population was aged 65 and over in 2010. This is projected to increase to 19.5 percent in 2030. "Thailand's aging demographic is very serious and there is a sense that this is not being discussed at the policy level," said HSBC ASEAN economist Su Sian Lim.

"If you look at the U.N.'s population projections, the working-age population will decline as early as 2017 – that has very significant growth implications," she said. "The productive part of the labor force is disappearing." An aging population can have important long-term consequences for an economy – from pressure on state finances and the health care system to lower economic growth rates if the workforce is not large or productive enough to support retirees. Old man According to BofAML, Thailand is the only country in Southeast Asia that will join the ranks of Northeast Asian countries such as China and Japan in seeing their working-age population shrink over the next decade. (Read more: China's solution to elderly neglect: Sue your kids) In short, BofAML analysts believe Thailand has emerged as the "old man" of Southeast Asia. "We have been used to seeing developing economics such as Thailand growing at annual rates of 5 to 6 percent – that growth would be in question with an aging demographic," said Seng Wun Song, regional economist at CIMB Bank in Singapore. Economists attribute a successful contraception program introduced in Thailand in the 1970s for the falling birth rate, which means fewer working-age people in the future. Thailand's fertility rate declined to 1.6 percent in 2011 from 2 percent in 1992, near developed peers such as Singapore, which has a fertility rate of about 1.2 percent, and well below levels seen in the Philippines and Indonesia. (Read more: For Western retirees, the Asian lure is hard to resist)

It's not the only country grappling with an aging society: Japan, China, the U.S. and much of Europe is in the same boat. The Singapore model