Actually, it's master up until 13. I got married recently and did a little etiquette research on this names thing. The kids loved getting their own invitations with master printed on them They thought they were royalty or something. I think miss is used a little longer for girls though. I can't imagine calling a 13-year-old girl "Ms."!
Or if you want to be really technical, it's Miss Lastname for the oldest(unmarried) daughter, and Miss Firstname Lastname for the younger daughters, and collectively, The Misses Lastname.