How do weebs not all know this by now - the low age of consent in japan is so teens arn't commiting a serious crime sleeping with each other, if an adult does it it's a different matter, they have decency laws on top of the age ones that effectively even mean both partners can have to be over 21 to be totally legally in the clear in some places. It's not a thing really at this point but is meant for things like engaged teens and so the rural sorts who married young wouldn't be commiting a crime, to not be breaking those laws she would need to be in a "socially acknowledged relationship" / basically need his parents and schools permission. Doing it that way as two seperate laws makes more sense imho than a single cut off age of consent that goes straight to one of the most serious of crimes and sentencing as a technicality within a teen relationship where the couple are within a months age of each other.