Most dentists want to see them when they're 3...DS had an appointment a couple of months ago...we took the girls with us so they could see what was going on (they had just turned 3)...our dentist said to just bring them in the next time we bring DS in - which will be in November...they'll be 4 in Feb. I think if they're too young, they won't cooperate and it will be useless anyways.
We tried taking DS when he was 3, but it was a disaster...he was 4 when we finally got him to calm down enough to have them look at his teeth...now he does just fine!!
Unless there's a concern, I'd wait until they're a tad older.
I agree w/ the pediatric dentist - so much better than a family practice (even when they're friendly and good). At our dentist, it's all about making the kids feel comfortable.