NY Newsday is carrying an AP article that suggests the audience level for ROTS and why. Here's a clip, head there for more:

SPOILER WARNING!

Anakin is left gruesomely mutilated in a death duel with former mentor Obi-Wan Kenobi (Ewan McGregor). His surgical reclamation as the part-flesh, part-machine Vader is chillingly juxtaposed against the bleak childbirth scene of his wife, Padme Amidala (Natalie Portman), whose twins grow up to be heroes of the original "Star Wars" trilogy, Luke Skywalker and Princess Leia.

Even cute and cuddly Jedi master Yoda takes his lumps, torched by the electroshock treatment dished out by the saga's puppetmaster, the evil emperor (Ian McDiarmid).

The previous five "Star Wars" flicks all were rated PG, which carry the mild warning "parental guidance suggested," and that some scenes might be unsuitable for children. The PG-13 rating carries the alert "parents strongly cautioned" that some material could be inappropriate for those younger than 13.

"We're getting a lot of flak from parents, a lot of people saying how can you do this? My children love these movies. Why can you not let them go see it?" Lucas told The Associated Press in a recent interview. "But I have to tell a story. I'm not making these, oddly enough, to be giant, successful blockbusters. I'm making them because I'm telling a story, and I have to tell the story I intended."