High school seniors are 17
I think theyre just translating the author's notes so they left the age the same. The author is using the Korean age system. At birth everyone is 1 year old and then everyone gets another year added on Jan. 1. Since his birthday is Jan 2nd, and not later in the year like December, then he would be 18 in international age I believe, not completely sure about that tho.
To add on this is why you'll see a lot of manhwas list higschoolers as 18-19 year olds despite them actually being around 17-18 internationally.
I thought I heard people say that the vampire boy (I can’t be bothered to remember names right now) is 17, but he’s 19? Since this is the official English translation, is this correct?