Is the under 13 audience that large to make film makers change their movies to achieve the PG-13 rating?

It’s the 13–17 demographic they are after, not under 13.

Most movies are geared to attract the largest possible audience, so, yes, many films are cut to achieve the PG-13 rating in the US. Avatar, for example, had a sex scene cut for theatrical release that will be restored on the DVD.

Plus, most parents won’t let their kids go to see an R movie. Not that they won’t see it anyway, but the intention is for kids not to see such films.

I think they are looking to attract families with “family values” with a PG-13 rating. Parents who want to know a film is safe to take their pre-teens to.

Take is the operative word there.
While there aren’t a lot of kids going to the movies, there are a lot of families that go together, and that is BIG money. Try to take your kids to the movies without popcorn, sodas and twizzlers.

I think it’s about the audience who has the free time & money to throw away on movies. Adults have jobs & bills to pay & a home to take care of. They are a bit wiser with their spending. Pretty sure my parents haven’t gone to the movies since the last Borne movie. They have home projects to do & other things to get done. Taking 2 hours out of their day to watch a movie they may or may not enjoy doesn’t appeal to them.

Anything that can be successfully marketed to kids is a gold mine. Think of the layout of the grocery store, with all the healthy cereals on the top and bottom shelves, with the sugary crap cereals right in the middle where kids can see them and beg for them.

Having a movie rated PG-13 just opens up that much more of a target audience. If, for example, Avatar had been rated “R,” it wouldn’t have been advertised on Nickelodian or in Happy Meals. My five year old is still mad at me because I won’t let him watch it. I can imagine that there are a lot of parents who give in and take their kids to see PG-13 movies even if they are under 13, just because they feel like they can’t compete with the way these movies are aggressively marketed to children. Maybe there are others who think there isn’t much difference between PG and PG-13. It actually really annoys me that so many movies have advertising aimed at kids under 13 that are inappropriate for that age group. Batman, Spiderman, Iron Man, Indiana Jones, Avatar, all show up in toys, commercials, cereal boxes, and Happy Meals that are primarily aimed at younger kids. I get a little sick of explaining to my kids (the oldest is 11), that they can’t come to Iron Man 2 with us because it isn’t a kid movie. Everything they see tells them it is a kid movie.

Yes, it’s that large. Here’s an example of a typical family outing: divorced man goes to pick up his kids for evening out together at dinner and movie. The kids are 10, 14 and 15 so what kind of movie do you think dad will pick out? The answer is going to be whatever will be okay for the youngest and everyone else agrees to it.

It’s a large enough segment to matter, plus a family with a kid in tow is not going to select a movie that isn’t appropriate for the youngest member of the group.