The people who are most vulnerable to depression are those in their 20s and 80s.
Many in these age groups are not getting treatment, either because they fail to spot the symptoms or because they are embarrassed.
The number of patients who have been treated for depression rose more than 2.2 percent annually, from 587,860 in 2012 to 641,987 in 2016, according to the National Health Insurance Service.
But a closer look shows that the number of depression patients in their 20s soared six percent on average and among over-80s 12.8 percent, while the figure in other age groups was more or less flat.
"Twenty-somethings suffer greatly from psychological fatigue because they have to be responsible for their own lives for the first time," says Prof. Lee Hae-guk at St. Mary's Hospital in Uijeongbu. "Many suffer from low self-esteem when they start life at college or in a company."
As for the elderly, depression often spreads because they live alone and lose contact with their family, says Prof. Na Hae-ran at St. Mary's Hospital in Seoul.
"The older you are, the more you tend to think of diseases or death and become obsessed with negative thoughts," said Chae Soo-mi of the Korea Institute for Health and Social Affairs. "This can increase the risk of depression or other mental illnesses."
An inspirational message is hung on Mapo Bridge in Seoul on Jan. 23. /Yonhap