I think it actually is illegal everywhere but Japan. Mind you their age of consent is 12, so it is not quite paedo stuff for them.

Their laws are a tad complicated but fair I think though, I heard that if you are 40 and bang a 13 year old girl, you are effectively going to get thrown in the nonce van en route to getting your dick chopped off in prison.

BUT... If you are a 13 or 14 year old boy shagging the same girl though, you are not only going to avoid getting arrested but probably applauded for a job well done too. Somebody with Japan experience would need to confirm this.