is he the boy from the beginning? cause now I'm confused

yeah he is!! the first chapter is a flashback 4 years before the current timeline and the boy (jeongwoo) in chapter one is now a freshman in university, and the big dude in ch 1 (eungi) is a senior in university

korean age is +1 or +2 years from international age so just subtract that from 20 & 24 and those are their ages