Ok if lets say a young child at the age of like 6-7 was allowed to play grand theft auto by his/her parents maybe the child would understand incorrectly and start doing what the people do in the game this may cause to serious trouble jail time or even death. In this case dont let your kids play underrated games or watch underrated movies.
Hope that helped you understand more.
I don't believe that there are rules.
Hope I helped,
Ms. Weasley