The report also showed that 45 percent of people aged between 13 and 24 said they suffered from stress in 2018, down from 46.2 percent in 2016, with stress from work and school being the main contributors.

South Koreans aged between 13 and 24 were most worried about their future job, followed by academic performance and physical appearance, according to the report.

The report said that 27.1 percent of middle and high school students had depression in 2018, down from 38.8 percent in 2008.

It showed that 6.7 percent of teenagers in middle and high school smoked in 2018, up from 6.4 in 2017, with 16.9 percent having drunk some form of alcohol, also up from 16.1 percent in 2017.

According to the report, those in the 6-21 age group reached 8.04 million in 2019, accounting for 15.6 percent of the country's total population. The ratio is expected to further fall to 9.8 percent in 2060, the report estimated.

The 2019 ratio marked the lowest level since the related data started to be compiled in 1970, when it stood at 39.1 percent. The sharp decline is blamed on fewer marriages and the country's chronically low birthrate amid an economic slowdown.

The number of South Koreans aged 65 years or older could account for 20 percent of the population in 2025 and surpass 40 percent in 2051, compared with 13.8 percent of the country's population in 2017, according to Statistics Korea.

A country is defined as an aged society when more than 14 percent of its people are 65 or older.

Among school-age children, those from multicultural families reached 122,000 in 2018, up 11.7 percent from the year before. That is equal to 2.2 percent of more than 5.59 million students in the country in 2018.

Internet use among teenagers edged up last year. Teenagers spent an average of 17 hours and 48 minutes per week on the Internet in 2018, compared with 16 hours and 54 minutes in 2017.

The report said 29.3 percent of teenagers were heavily reliant on smartphones in 2018, compared with 30.6 percent in 2016. (Yonhap)