RedFive said:
PG-13 wasn't adopted till '84, thanks to Spielberg
http://en.wikipedia.org/wiki/Motion_Picture_Association_of_America_film_rating_system#The_PG-13_rating_is_adopted
Exactly.
What bugs me the most about all this is that, while PG-13 was intended to split the PG category between the less violent films and the "maybe-possibly-should-be-rated-R" films. Unfortunately, what has actually happened is that PG-13 has essentially replaced PG, and PG has taken G's place, while G has become largely irrelevant, with a stigma almost as damaging as NC-17 (though with G, the stigma is "it's for little kids, not adults" and not "it's porn"). No one would see a movie like 2001 today if it was released with a G rating (which it was in 1968) - they'd send their kids to see it, sure, but they'd think "Oh, it's rated G, it's made for kids, not me."
Frankly, I think the whole CARA system should be abolished, and replaced by a voluntary "rating" imposed on the film by its director or producer. I.E., for WALL-E, the poster/advertisements would say "This film is appropriate for family viewing" (the implication being that it may be a little intense for little kids to watch on their own, but should be fine for kids watching with their parents), whereas something like The Dark Knight would say "This film is intended for teenage and adult audiences, and is likely to be too intense for younger children."
Not perfect, I know, but I just hate the way the MPAA/CARA works.
Also, anyone interested in the topic should check out the documentary This Film Is Not Yet Rated. Very good, and very enlightening.