I took K in at 3 or 4 when she fell and broke off a tooth, took her to a regular childrens dentist and they wouldn't even touch her. They gave me a list of pediatric dentists and I picked one and have never looked back. She went to sleep, had all her dental work done in a few minutes, woke up and wanted to go eat. Never cried, never complained about anything.
J went at 20 months and was so enthralled with everything he sat the entire time they were cleaning his teeth that he nver moved and just kept smiling at the pretty ladies.
I think the younger the better, that way they have a positive experience before they may have a cavity and they have that trusting relationship. I would never have dental work done on a child and them not be asleep during, just my opinion.