It's very simple. 18-30 year olds regularly go to the movies. More often than any other demographic group.
As one friend put it, "They'll see anything," which is only partially true, but he has a point. They go out to a movie almost every weekend.
The biggest market out there is the "family" audience, but they don't go to see everything released. If you can make a good (enough) family film, you'll be golden. You can pull in everyone. But if you're movie "kinda sucks" they will stay away in droves.
Appealing strongly to the 18-30 year old crowds (actually it's 18-25), you've pretty much got a solid chance at a guaranteed audience.
It's the closest thing to a "sure bet" in Hollywood (which has no sure bets), and when you're risking millions of dollars and many future jobs, it pays to target that audience.
Even if they have no taste and will go to see "crap," time and time again.
"Jaws is the Citizen Kane of movies."
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