I think it's weird how Hollywood changes things. The girl's real name was Annaleise something and she was 23 when the culmination of her "possession" took place. I wonder if the movie will go into it but she had relapses and periods of this kind of behavior between 16 and 23; so if the movie just says it's this one terrible episode, that will be incorrect.
Annaleise's story is so sad though
I kinda think it's wrong of them to make a movie out of it, actually.