Does anyone know how old Bum and Sangwoo really are ?

I think they're in their early 20s since they both already did the mandatory year of military service and Sangwoo is a college student

Yeah I know that but I would like to know there specific age
Either way they died young

From reading the first chapter, at the start of the story, Boom was either 25 or 26. Sangwoo was 21 or 22 (21 is more likely). On top of that, I think they only spent about a year together in Sangwoo's house cause I only remember one winter (the ski trip).