I don't know how it is done in other countries, but in America the R rating allows anyone under 17 to go see a movie as long as they have someone 17 or older going with them. It's only the NC-17 rating that doesn't allow anyone under 18 to see the movie no matter what.
The PG-13 rating is the general everyone can see it rating. It can be violent and sexy, but not too much so. Most films are rated like this these days, especially blockbusters.
G and PG ratings are now considered for only kid and animated movies by the public.