The sooner the better!
At 3 and 5, my little ones love going to the dentist. They have had their teeth counted, flossed and polished. Having been since they were 2, they are both very comfortable with the whole experience.
Having the hygienist chat with the boys about how and when to brush their teeth and the importance of a good diet also reinforces what we teach (and sometimes get resistance to) at home. They also walk away with a goody bag (usually a toothbrush and a tube of toothpaste) which is a lovely and exciting bonus.
Taking Ds2 so young turned out to be beneficial as he has a condition that means his teeth are more susceptible to cavities. Having this info early has been advantageous.