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.
In Italy you have just three ratings:
T (Per tutti – All): All ages admitted.
V.M.14 (Vietato ai minori di 14 anni – Restricted to 14 and over)
V.M.18 (Vietato ai minori di 18 anni – Restricted to 18 and over)
It's much simpler than the U.S. convoluted rating system
Some titles for comparison:
Title - US Rating - Italian Rating
Cars - G - T
Matrix - R - T
The Wolverine - PG13 - T
Aplocalypto - R - T but re-rated VM14 after appeal
Last Tango in Paris - NC17 - VM18
Zack and Miri Make a Porno - NC17 - VM14
Really, do you need all these ratings?!?