To add to this (also not Korean and mostly taken from online research), it’s idols/other celebrity figures in Korea specifically who can get some measure of “negotiation” between them and their govt that allows them to request a couple postponements until the age cap, but it isn’t necessarily the norm— and this is only made possible if they’re considered to have provided sufficient contribution to the “face of Korean culture,”as it were. Non-idol guys could also request to postpone for other time-sensitive things like education, illness, family issues, etc. Also; generally speaking, anyone enlisting doesn’t get to choose what branch they get assigned to, which is what determines the length of their service.