Teenagers are already aware of the topics which movies receive R ratings for. Movies receive R ratings for three main reasons: Adult language, adult situations, and extreme violence. These are things that are known about and in some cases even seen in everyday life by the time you are 16. Many people use "adult" language now. Most teenagers even use it and those that don't use it hear it constantly. "Adult" situations, generally sexual, are not an issue by the time somebody is 16. In an R-rated movie these situations are implied and not shown. Sex education is taught in public schools, so 16-year-olds are very aware of sex. Violence is shown all the time on television and in video games, which 16-year-olds can see anytime. Seeing these things in a movie won't make them any more or less aware of them and likely to do them.
The current policy on 16-year-olds and R-rated movies is intended to help parents screen what their children see; however this isn't the case at all. Parental consent is not required, only that an adult attend the movie with the 16-year-old. It can be any adult. Most 16-year-olds have 18-year-old friends who could attend a movie with them and get them in. It is the parents' job to know where their children are, not that of the theatre or the government. This policy merely gives parents a false sense of security.
Sometimes R-rated material is necessary in movies for artistic reasons. Just as a woman in a movie set in the 1600s wouldn't wear blue jeans, a street gang must use street slang and language