I think you are looking to much into this. He's what 22? and pissed, people say things like that all the time as they can't imagine themselves being 90 and reliant on a nurse to feed them. Also Sangwoo lost both his parents when they were relatively young, so doesn't really have any close relations who are older adults anymore and he is more aware than most of death rather than this "I'll live for ever, nothing will hurt me" attitude some younger people have.
Maybe he plans to kill himself before he turn into an adult
He is an adult though. He's a grown man at university, having a job, paying taxes, driving, and has completed his military service.
Before he's old* he's already an adult... But he was just drunk (≧∀≦)
But the way he's treating yoonbum makes no sense.. sudden change in character after the police scene is too suspicious..maybe from now on he'll bring victims and bum will kill them for him
See how sangwoo says I will never grow old?.. he'll probably take control of bum and manipulate him completely. To continue his work and in the process kill sangwoo ╥﹏╥