Researchers unveiled a population clock predicting that Japan could theoretically become extinct in 1,000 years due to sharp decline in birth rates.
Academics in the northern city of Sendai said that Japan's population of children aged up to 14, which now stands at 16.6 million, is shrinking at the rate of one every 100 seconds.
Their extrapolations pointed to a Japan with no children left within a millennium.
"If the rate of decline continues, we will be able to celebrate the Children's Day public holiday on May 5, 3011 as there will be one child," said Hiroshi Yoshida, an economics professor at Tohoku University.
"But 100 seconds later there will be no children left," he said. "The overall trend is towards extinction, which started in 1975 when Japan's fertility rate fell below two."
Yoshida said he created the population clock to encourage "urgent" discussion of the issue.
Another study released earlier this year showed Japan's population is expected to shrink to a third of its current 127.7 million over the next century.
Government projections show the birth rate will hit just 1.35 children per woman within 50 years, well below the replacement rate.
Meanwhile, life expectancy -- already one of the highest in the world -- is expected to rise from 86.39 years in 2010 to 90.93 years in 2060 for women and from 79.64 years to 84.19 years for men.
More than 20 percent of Japan's people are aged 65 or over, one of the highest proportions of elderly in the world.
Japan has very little immigration and any suggestion of opening the borders to young workers who could help plug the population gap provokes strong reactions among the public.
The greying population is a headache for policymakers who are faced with trying to ensure an ever-dwindling pool of workers can pay for a growing number of pensioners.
But for some Japanese companies the inverting of the traditional ageing pyramid provides commercial opportunities.
Unicharm said Friday that sales of its adult diapers had "slightly surpassed" those for babies in the financial year to March, for the first time since the company moved into the seniors market.
Unicharm started selling diapers for babies in 1981 and those for adults in 1987, said spokesman Kazuya Kondo, who declined to give specific figures on the sales.