Yes. I'll try to explain the timeline as simply as I can but I'm sorry if it's still confusing.
1. In the first scene (when MC quit swimming), MC was 17 and ML was 11.
2. At the Olympics there was a time skip of six years, which means MC was 23 and ML was 17.
3. Another timeskip happens after that (the morning after MC had a one night stand with that dancer). It is very implicitly stated that 4 years have passed since the Olympics, which means that MC is 27 and ML is 21. This is the current timeline.
As to ML's age of 19, it was when his scandal with the Taiwanese water polo athlete happened during the Asian Games, but it was brought up again in the news because it had something to do with his current scandal with that actor (Cha Seonghyeon).
Hope this explains
why does Jaekyoung age keep on changing? First it was 17 then 19 and now 21