I have often asked and wondered the same thing.  My SWCAH son had all his baby teeth by the time he was 9-10 months old and lost his first 2 bottom teeth by the time he was 5.......I have heard this is not unusual however and that as the other moms have said, all kids are different.  My SWCAH daughter is now 5 and no sign of losing teeth yet at all.  Go figure!!!!!!