From her blog:
Yes, the boy Jonas will be older in the film than the book...as will his friends Asher and Fiona. Some readers are already annoyed (in some cases outraged) about that. Entertainment Tonight apparently says that I should be, too. I'm not. A book and a movie are not the same thing. A movie needs visual stuff, and action; and this is a laregly quiet and introspective book. In the film there will be things happening that are much more believable if an older teenager is doing them...the escape of the boy and the baby is much...what would the word be...bigger, vaster, harder. Epic.