The math doesn't seem to add up, In Ch2 Pg8, Haruto is 5 years old. Ch3 Pg1 is a month after his Reincarnation, meaning he's still 5 years old. Ch3 Pg11 is 5 years later and the first day of magic school, which would make him 10 years old? But given the how old kinda looks, I'm guessing this was a mistake and either he is supposed to have reincarnated (or awoke his memories) at 10 or it has been 10 years since his first kill, making him 15 years old. And assuming Haruto (a first year student) is 15 years old, that would make the 7th year students, 21 years old.