The 20s of this generation are literally the ones that are getting the shorter end of the stick. Imagine yourself being 18 year-old male in South Korea, 80% of highschool graduates go to college, so given the status of the job market you are pretty much forced to go to college unless you are willing to work in a poor environment (as in jobs that don't provide career progression, or pose high risk on personal health).
To add salt to this wound, the college tuition has been hiking. The parental generation of Korea who are in their 50s lived through a time of high economic growth, and education was affordable due to cheaper tuition that could be paid off in short term. Let's say that you take college somewhat seriously, and spend extra year or two after highschool to get into a solid college. You are now 20. After a year you have to serve the military. You are now 23-24, second year in college. After finishing college, you are 26, but there are good amount of people who take year or two off so you will get plenty of 27-28 year old males that have lot of debt, and not much job experience.
Ok, time to get a job. But hey, the job market sucks right now. Older people aren't retiring. Some companies give you contract or internship work but these barely pay for cost of living, and rarely convert to satisfying full-time positions. There were times when college degree meant a guaranteed full-time employment but not anymore. If you managed to land a full-time job after gaining job experience, you're probably 30. To make things better, real estate is spinning out of control so you wouldn't be able to afford a house any time soon. If you asked a person like this about getting married and having children within a year or two... You can't blame them for saying no.
Population kept scaling with the assumption that economic growth will match it, but things have slowed down. It's not like Koreans will go extinct or kill themselves as a whole. But it's the matter of how much is the population going to drop by and whether they can find breakthroughs in the meantime to grow its economic capacities.
As a side note, although people are mainly concerned about teens or young adults when discussing suicide issues, I would like to point out that suicide rate in Korea is exceptionally higher for older people (60+).