I'm 26, and for me the movie is iconic. I think they're in a good position where the ticket buyers are slightly older now and can afford tickets on their own. The movie is also still popular, and many younger kids I'm sure convinced their parents to get tickets for them. I'm not shocked at all by it's popularity.
Also, the St. James has 500 more seats than the August Wilson. MEAN GIRLS picked a smart house.
"Oh look at the time, three more intelligent plays just closed and THE ADDAMS FAMILY made another million dollars" -Jackie Hoffman, Broadway.com Audience Awards