In one of the later chapters, Heejae and Garam spoke about when Heejae debuted? (i think, i don't remember if its his debut year but it was something to do with 2003) in 2003 I think and garam spoke about in 2003, heejae was garam current age and heejae joked that in 2003, garam was 3 years old so it's leads to believe that Garam may be born in 2000 or 1999.
Im sorry I can't really remember the exact translation so I'll update tomorrow if I remember lol.
I'm pretty sure it was mentioned in one of the comment section on here on garam age so if you go back a few pages, you can find it.

How old is the professor?