Speaking at the seventh annual Ho Rih Kwa lecture organised by the Singapore Management University, Mr Lee said: “Based on trends, if we project into 2050, even with immigration, the population pyramid will be inverted ... We are going to be growing old faster than any society in the world.”

He added: “Who will pay the taxes, to spend on whom? How do we keep (the economy) prosperous, vibrant and forward looking? Who will man the Singapore Armed Forces and defend us? We can’t be the Dad’s Army.”

Mr Lee noted that the Republic had enjoyed “favourable” demographics for the past five decades, with a post-war baby boom. Fortunately, the nation was able to educate the baby boomers, create enough jobs for them and make them “valuable assets” contributing to Singapore, he said.

“Had we not been able to do that, they might have ended up (as) migrant workers abroad, working in tough jobs with low wages, because the alternative of staying at home was even worse.”

Between 1965 and this year, Singapore’s population grew from 1.9 million to 5.5 million. However, the number of citizens aged 65 and above is increasing rapidly, as population growth slows. The size of this group of citizens doubled from 220,000 in 2000 to 440,000 today, and is expected to increase to 900,000 by 2030.

Last year, the total population here grew by 1.3 per cent — its slowest pace since 2005, while total fertility rate continued to fall despite the Government’s incentives and encouragement, said the Prime Minister.

As a result, there has been a rise in “sandwiched” families, where two or fewer working adults are supporting both younger and elderly dependants. There is also more demand for healthcare and social services, Mr Lee said.

The Government has responded by helping seniors “age well and live fulfilling lives” through home ownership and good healthcare, for instance.

Other policies such as the Central Provident Fund and Silver Support Scheme, which provides permanent handouts to eligible seniors, are also in place to provide them with a good foundation for retirement, said Mr Lee.

By 2050 this is set to increase to two-thirds. By every measure, Japan has the oldest population - not just in Asia, but globally. Singapore is not far behind with ageing populations on a par with Europe where the challenge of the ageing population is widely recognised and hotly debated.

With the highest median ages, longest life expectancy rates and among the lowest fertility rates in the region, Singapore is already facing the immediate challenges of declining populations and talent shortages.

At current birth rates and without immigration, Singapore’s citizen population will start to age and shrink. Between 2011 and 2030, the median age of the citizen population will rise from 39 years to 47 year

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