@jameaney @Huichelaar
Age of consent in Japan is 13. Also:
https://en.m.wikipedia.org/wiki/Age_of_consent#Gender_of_participants
There's nothing wrong (ethically) with the whole situation even if the MC was not 40+ actually. It's more than obvious he willingly has sex with her. You can argue how he's too young and consequently mentally deficient to give his consent, but I think you will be wrong.
https://en.m.wikipedia.org/wiki/Age_of_consent#Gender_of_participants
There's nothing wrong (ethically) with the whole situation even if the MC was not 40+ actually. It's more than obvious he willingly has sex with her. You can argue how he's too young and consequently mentally deficient to give his consent, but I think you will be wrong.