It’s just an art, just don’t read if it bothers you.
Yeaaa he looks like a middle schooler but ig the seme looks young too so maybe it's a art style?
I think it's art style in this case;
Everyone looks pretty young here (in my opinion) ┑(￣Д ￣)┍
And There's worse cases where a certain characters looks wayy too young (cough cough Siren's song) but i really don't think that's the case happening here, I can still see the mc as 21 just in a cuter artstyle (●'◡'●)ﾉ
Ker-nim's art style is just like that. If you have read Sign, even in that it is the same case. It is just a bit more polished version of the art style in Sign. So don't worry
I loved sigh don't get me wrong but the bottom looks so young it's a little weird