My son is 3 1/2 and SUPER causious of new people. My ped said unless he notices a problem before hand, not to worry about the dentist until 5.
That waid, I had a dentist appointment about 2 weeks ago and he and Dad came with me. He watched what the dentist was doing to me and allowed her (after about 30 minute, and with the promis of a toy) to "count his teeth" in order to provide a visual exam of his teeth. She said that he seemed okay and sugested that DH and I schedule our appointments every 3 months (so DH is supposed to go in Sept, me in Dec...) and bring him in and ask every time if he's ready to let the dentist brush his teeth like they do to ours. If by March, he hasn't volunteered readiness, to schedule a June cleaning for him next year (at 4 1/2). She felt that his ability to cooperate was developed enough to be able to handle it.