Out of curiosity I looked up what the age of consent is in Japan, and it's 13! lol, but with parental permission in most prefectures, in Tokyo is 18 without it. So waiting till he's 20 is legally unnecessary. He doesn't need to reach adulthood(currently 20 years old) to have sex with her.
@CBFlap @AerospaceGlobe She didn't know how he felt, the safest guess it that he's not been serious. She doesn't want to get delusional and get her hopes up for their relationship if he doesn't have them too. But this is just an assumption based in my personal experience, could be something else.
Regarding her wish, anyone have an idea of what could it be? Maybe this https://mangadex.org/chapter/302709/2 ? That would be to simple, i think. But it would explained this part https://mangadex.org/chapter/967153/16 .