Or maybe he uses magic that makes him old
also could be possible that he look daunting rather than old due to all the stress he face but author-san art style making it look old instead
Imagine caring for 13 mischievous demon (or more)
Imagine having a principal lika ojii-san and having a student like iruma who cursed at the very first day
or maybe he's just too stressed bro... being a teacher is a stressful job
Kalego looks too old compared to his older brother father and uncle, I think there's a story behind that like he traveled in time or was in a place like the time room in dragon ball