The report by the Australia-based Macquarie Securities Group said the proportion of people aged 40 years or younger to the entire population here plunged to 48.1 percent last year from 69.4 percent in 1995.
The percentage is expected to further decline to 32 percent in 2050 due to a continued drop in the birth rate while the population's life expectancy continues to grow at a fast pace.
South Korea has one of the lowest birth rates among the members of the Organization for Economic Cooperation and Development while the number of newborns here dropped for two consecutive years in 2013 and 2014, according to earlier data from Statistics Korea.
The country is already an "aging society" with 7 percent of the population over 65, and will become an "aged society" with 14 percent of all people over 65 in 2018. It is expected to become a "super-aged society" in 2026, when one-fifth of the population will be over 65.
The Macquarie report noted that Japan also faces similar issues of an aging population but that its outcome will be quite different from that of South Korea.
In fact, the aging of the population in Japan may even have some positive aspects as many people there continue to work past their retirement age, making them big spenders who help boost consumer spending, the report said.
"The Asian market will walk a path to either progress or retrogression over the next 20 years depending on its actions now," said Hwang Chan-young, head of Macquarie Research.
"Population and consumer spending, along with changes in China's economic structure and technological development will become crucial challenges and opportunities that will decide the future of Asia over the next 20 years." (Yonhap)