I haven't heard anyone use "master" in years, probably it's un-PC? But I read in an etiquette column recently that you use "master" up to about age 7 or something, and "mister" after age 12, and from ages 7 to 12, no title at all for a boy. I may be misstating the ages, but it was a weird complicated rule that I'd never heard of....
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."!