The use of 'girl' to refer to women often bothers me as well, but in this case, it's a reference to a song title: http://en.wikipedia.org/wiki/It's_Different_for_Girls
And, she does call herself a girl once more, not just in the title. Perhaps someone said that sentence in a movie, so it will somehow not mean what it says either.
And even if it is an unspoken reference, that doesn't explain how the title should be interpreted differently. It's a pop song, and according to wikipedia the lyrics aren't especially relevant to the contents of her post.