where is the age of the protagonist? I would say, judging by the otaku-kun, that he is by no means less than 16 or even 17 years old - the age when you can fuck the whole city and still remain in reserve for a small village.
let's go further - according to Japanese law (and I assume that this story happened in Japan), teenagers from the age of 18 are admitted to higher educational institutions. that is, the main character is at least 18 years old. from here we draw conclusions. where did you get that the protagonist is 13?