Why the hell did the author had to include twice that one of their members was 13! Just feels unrealistic given all the combat and slimy as there is supposed to be romance.
13: Because of age of consent in Japan is 13. (Though that's national, some prefectures (e.g. Tokyo) effectively raise it.)
(Today, I am sadly reminded that there's entire website devoted to "age of consent" around the world. Though at least it provides answers to questions like this.)