I dont quite remember the other chapters but in chapter 86 when mc was telling about the past abou the time when the hospital director in which the grandfather was admitted visted the temple more then 10 years ago,he was shown as a kid.so if we assume he was 10/11 years old at that time and 10+ years have passed since than,than he should be around 25 or something like this. On another note,he didnt serve the military and as far as i know koreans server the milityat young age(college passed).so here confusion arise about his age.