i think it should be rated pg-13 because the filmakers should know the age of some kids that watch twilight, so they shouldn't make the movie an adult rated movie.
Well seeing as though Stephenie Meyer is a Mormon and tends to shy away from nudity and obvious sex references, as long as the movie sticks to the book i cant see why it should be rated highly!
they have to make it pg13 so all the fans can watch the movie. i dont know that many twilight fans over 18 (i only know one, my uncle's ex who wasnt really that into it anyway)