Another thought is injury to the tooth. My daughter's tooth turned a little ashen after two hard bumps within a few days of each other. Her tooth also loosened up a little bit. We told her not to bite into ANYTHING for a few days, and that seemed to help. It tightened back up and colored up after a few days of babying it.
As far as losing teeth, I've always heard that if your child got teeth as a baby early, it stands to reason that they will probably be the first ones in their peer group to lose their baby teeth. My kids both got teeth later as a baby, so they will probably be the last ones in their classes to lose them.
J.