Not the guy you are replying to but... I said it in a previous post, it's pretty clear that it's actually an military boarding school for teens but the author had to make them all 21 to comply with patreon rules. I don't think the MC looks 11 but 13 to 15 is well within range, the rest is artsytle. The fact that he is socially awkward about getting a boner is classic teen behavior and he talks with his two childhood friends as if they were in elementary school within recent memory.
Hogwarts went from pre-teen to young adult while PH seems to be 13 to 21, that is a noticeable difference but still a school for kids rather than ROTC.
What gave you the impression that it was serious military org for 18+ people?
Because I got the total opposite impression within 5 minutes. There is a clear separation in age groups with upper classmen appearing noticeably more developed (young teens VS young adults), characters talk about schools clubs and fun sporting events all the time (photography, fencing, biking, nature hikes). All the students you interact with act like young teens, one of your first quests is putting up posters for what amounts to a school club for a bratty young girl that acts like a spoiled princess.
If you look at the story objectively it is just Hogwarts with a military theme.