@Zhiyu
I'm late but it's just the translators fault he's 19! The setting off our story is in South Korea the age off consent in South Korea is 20 in the beginning when he said he was not off illegal age that means he wasn't 20 but the translator changed it to 18 because that's how the age off consent works in US but the boy is 19 Spoilers ahead
in chapter 24 you will see him say he's 20 !