For South Korea, the number of people under 15 will drop 11.5 percent, as well as 25.5 percent for the 15-19 group, 33.5 percent for 20-somethings, 29 percent for the 30s, 18.8 percent for the 40s and 11.9 percent for the 50s. By comparison, the population for the 60-64 group is set to increase 23.5 percent. For the over-65 group, the increase will reach 118.6 percent, contracting the size of the working-age group to 18.9 percent of the total population.

The working-age group in the country peaked at 37.63 million in 2016 and started to wane last year, a result of people born in 2002, when the birthrate plunged to 1.17, entering the age group.

The group accounted for 73.1 percent of the total population in 2017. It is expected to go lower to 66.3 percent in 2027 and further down to 58.3 percent in 2037.

Professor Oh Min-hong of Dong-A University, who authored the report, warned that the shock to the local labor market will be deeper than in other countries because the population shrinkage for under-40 is especially stark.

"The population fall in OECD countries is moderately paced, and other age groups can make up for the diminishing group," he said. "But in South Korea, the entire age groups from the 20s to 40s diminish in great scale, so they cannot be replaced by people over 60."

The whiplash for such an aging society will appear soon, Oh said. 

"It won't be easy to reach a consensus on structural problems from the low birthrate and aging, so it's imperative that related parties start discussions now to find an active senior policy," he said. (Yonhap)