Editor: Hey, so the MC knows the hero is a woman…
Mangaka: Yeah, I thought readers would get tired of a gag of him being the only person not to know. Would be really hard to keep it plausible, y'know?
Editor: But if he knows she is a woman, than what's stopping them from getting together?
Mangaka: Oh, uh… Well, you see… It's because… she's… actually only 5 years old! Yeah!