A record-high 31.86 million people, or about a quarter of Japan, were over 65 years old as of Sunday, up 1.12 million from a year earlier, according to a demographic estimate released by the government.

The record high of 25.0 percent, up 0.9 point, was calculated based on births and deaths registered since the 2010 census, the Internal Affairs and Communications Ministry said ahead of Respect for the Aged Day, a national holiday that falls on the third Monday of September.

It attributed the growing number of senior citizens to the post-World War II baby boom.

The National Institute of Population and Social Security Research said it expects 1 in 3 people in Japan to be over 65 in 2035.

The number of men over 65 stood at 13.69 million, accounting for 22.1 percent of the male population, while women over 65 stood at 18.18 million, or 27.8 percent of the female population.

Those over 70 stood at 23.17 million, representing 18.2 percent of the overall population, while those over 80 stood at 9.30 million, or 7.3 percent.

Different data showed that a record-high 5.95 million people over 65 were part of the labor force as of 2012, or some 9.5 percent, also a record high, of all Japanese over 15. Of all seniors over 65, 27.9 percent of men and 13.2 percent of women remained part of the labor force.