Many people see TV ratings that are often presented in TV programs and always before movies. I know that not many people pay them any regard unless it's an rated R program, and they have particularly young children in the house. I know from personal experience that not many parents pay any real attention to this. My parents never seemed too. I remember being 12 years old and watching American Pie. I remember when an R rated movie would come out I wouldn't worry too much because I knew I could con my parents into renting it or buying it.
The biggest problem I have heard from people about the TV ratings is that a lot of PG-13 movies can be worse than R movies. The parents that DO trust the ratings have been upset that their children can go to a movie theatre in a trusted manner and coming out seeing more violent/sexual images than some R movies. This didn't surprise me because even some of my friends commented on this issue, and I agreed with them. Not that any of us were complaining, we just thought that it was particularly funny.