Yeah, the age thing is ridiculous. Teenagers in films are played by young adults all the time, and the original HALLOWEEN definitely features some "old" teenager boys and girls. If Danielle Harris' age actually bothers anyone, it's only because they are looking for reasons to complain about this film...the same people who would have spent the next seven months bellyaching that she was robbed if she hadn't been cast in the film. The girl could easily pass for 17, and her presence is a huge plus for the movie.
Frankly, I'd love to see a few of these "Isn't she too old?" folks say that to her face at a horror convention, instead of kissing her ass in person then turning around and talking shit about her on-line.