Speaking as someone from a Japanese background, the actual national age of consent in Japan is 13 but every prefecture has anti-obscenity laws that makes any adult (20+, will become 18+ once the law changes come through) that has sex with a minor (below 20, again, will become below 18) at the very least open to prosecution.
Actually correct sources:
https://www.ageofconsent.net/world/japan
https://www.quora.com/Why-is-the-age-of-consent-so-low-in-Japan