People are very protective about what children see now. Especially when children tend to have a very sharp memory. Here are a couple reasons:
pg13
Yes, New Moon is rated PG-13.
It has been rated pg13 so you have to be 13 years old
PG13
The movie is rated R.
It will be rated PG13 for adventure and violence.
No it's rated R
No, it's rated R.
No it is rated PG
No, its rated PG13.
PG13
Twilight wasn't technically rated PG-13 for it's language, more of the violence and blood that is shown in the movie. I would say it was right on the line between having a PG rating and PG-13 rating.