Truth be told in my opinion, age difference isn't actually that much of a problem IF the couple has already at the very least gone past the 10s age wise (not a kid or a teenager well until 17 would be actually), in other words both of them are 18+ years old cuz that would already classify them as adults and as such they can do almost whatever they want. In this case age difference is not the actual problem but that MC is 13-15 years old which would make the relationship if made public bring them a whole lot of trouble unless laws state otherwise and luckily for everyone Mari isn't even his teacher at school so he doesn't need to take precautions of being found out by the school.