yall are misunderstanding what's happening in the scene.. he doesn't mean he's been waiting for him to turn of age or anything like that. he's talking about when they meet again in college because he wasn't sure if wooyeon's feelings for him were real bc let's be real it's been 4 years and wooyeon's different from when he was 16 so when dohyun said he was waiting he meant he was doing everything he can to make wooyeon fall for him in uni so they have something solid to work from; and this is a spoiler but we get his pov later and he says he never thought he'd see wooyeon again after he quit tutoring him so why would he be waiting for him in that pedo way smhh
oof can't decide between AT or HTWMHO