HONG KONG, Jan. 18 (Reuters) - China's ageing population threatens key Beijing policy goals for the coming decade of boosting domestic consumption and reining in ballooning debt, posing a severe challenge to the economy's long-term growth prospects.

A record low birth rate in 2023 and a wave of COVID-19 deaths resulted in a second consecutive year of population decline, accelerating concerns about China's demographic downturn.

Large groups of the 1.4 billion people living in the world's second-largest economy will exit the labour pool and age past a prime period of their lives for consumption, exacerbating structural imbalances that policymakers have vowed to address.

Household consumption's share of economic output in China is already one of the lowest in the world, while many provincial governments - responsible for pensions and elderly care - are deep in debt as a result of decades of credit-fuelled investment-driven growth.

"China's age structure change will slow down economic growth," said Xiujian Peng, senior research fellow at the Centre of Policy Studies (CoPS) at Victoria University in Melbourne.

In the next 10 years, about 300 million people currently aged 50 to 60 - China's largest demographic group, equivalent to almost the entire U.S. population - are set to leave the workforce at a time when pension budgets are already stretched.

The state-run Chinese Academy of Sciences sees the pension system running out of money by 2035, with about a third of the country's provincial-level jurisdictions running pension budget deficits, according to finance ministry data.

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