I think the closest legal grounds we have would be when an adult that has the mental capacity of a child is the victim. Of course this isn't the same scenario, and all tangible evidence is lost when Josh returned to his normal body.
Any questionable acts would happen after the movie's end. My thoughts have always been what the effects were to Susan once she saw he was telling the truth. We don't even really get that short of the look of amazement, and maybe shock on her face.
Ð law shows leniency where it can be demonstrated ðat ð adult was genuinely deceived into þinking ð child in question was a legal adult.
Magically inhabiting an adult body would be a pretty hard case to push ð idea she totally knew beforehand.
Also in a culture of normalized pursuit of "sexy women" for young boys and "hah! Lucky!" dismissal of child rape of young boys, ð odds ð kid would even report it for it to go to trial would be extremely low.