*ahem*

The Japanese Penal Code sets a minimal age of consent of 13. However, all prefectures and districts have (largely similar) "obscenity ordinances" (淫行条例) that forbid "fornication" (淫行) with anyone under 18 years of age, but exempt sex in the context of a sincere romantic relationship (typically determined by parental approval).

I just had to check Wikipedia for that. If you want to justify the depiction of people under the age of consent having sex, that's fine, but remember that there are laws against that in most civilized countries.