TOKYO - A pair of 107-year-old Japanese sisters has been certified by Guinness World Records as the world's oldest living identical twins.

Ms Umeno Sumiyama and Ms Koume Kodama, who were aged 107 years and 321 days as of Wednesday (Sept 22), broke the record set in 2000 - 107 years and 175 days by the late Japanese twins Kin Narita and Gin Kanie, who were celebrated for their joie de vivre in their old age.

The duo, who now live in different care homes, were delivered their Guinness certificates as Japan on Monday marked its Respect For The Aged Day public holiday. On the same day, as per custom, Japan's Internal Affairs Ministry also released data on the elderly population.

A record 29.1 per cent of the 125.2 million population are now senior citizens aged 65 or older. Among them are 86,510 centenarians - half of whom turned 100 this year - in the world's fastest ageing country.

The government has emphasised that Japanese must embrace lifelong employment. This has translated not just in octogenarian politicians - Finance Minister Taro Aso turned 81 on Monday - but also in an ageing workforce where about 9.06 million elderly people - or one in four people over 65 - are at work.

This would be cause for celebration, if not for the risks present in the concurrent worsening fertility crisis, noted Mr Masashi Kawai, who heads the think-tank Research Institute of Countermeasures for Population Decline.

Even as people are living longer, government forecasts say that just 770,000 babies are expected this year. This estimation is derived from "pregnancy reports" that expectant mothers are to file at their city halls by their second trimester.

This will not only be the lowest figure since records began in 1899, but also a steep decline by about 70,000 babies from last year. It also marks a five-year decline after the number of childbirths slipped below 1 million for the first time in 2016.

The 770,000 figure had been forecast in 2015 as the case for the year 2036, which means the fertility crisis is now 15 years ahead of schedule with no signs of abating.

The bad news has triggered prime ministerial hopefuls from Japan's ruling Liberal Democratic Party (LDP) into action. The four contenders in the ongoing race to become LDP president pledged at a debate on Wednesday to set up a so-called Children's Agency to cut through bureaucratic red tape and unify policies to do with childbirth and children.

They also discussed measures like doubling the annual budget allocated to programmes for children, providing allowances for children of low-income households, as well as monetary support to have larger families.

Only former internal affairs minister Seiko Noda - who gave birth at the age of 50 after years of infertility treatment - had emphasised these policies in her candidacy.

But her other rivals have come round to the need to do even more as she painted a bleak picture of Japan decades down the road - with insufficient manpower for even its Self-Defence Forces.

Mr Kawai, who has advised politicians and bureaucrats on depopulation policies, warned that the decline in the productive population, defined as those aged 15 to 64, is already "the most pressing challenge" for Japan.

He lamented the inadequate results despite Japan having appointed a minister-in-charge of countermeasures to the declining birthrate since 2007.

Even recent policies, including monetary support for infertility treatments and childcare measures approved on Wednesday for fathers to take up to four weeks of childcare leave within two months of their baby's birth, are piecemeal and do not go far enough to shift the needle, he said.

He pointed to several structural issues, including many young people in unstable "irregular employment" with suppressed wages that add to the pressure of starting a family. And as many are marrying later, it will become more difficult to have larger families.

One other major concern, he said, was the decline in marriages by 16.1 per cent in the fiscal year ending March from the year before. Given that few children are born out of wedlock in Japan, Mr Kawai fears this would exacerbate the problem.