From experience, I have found that if you wait till the child is mature enough to handle a new experience, then it makes it easier and they want to go. Otherwise, if you take them when they are not ready, you can cause them to be afraid to go. With my kids, it was usually around the age of 4 or 5...each child is different.
By all means, if the child is having problems with their teeth, take them in! Just make sure that while you wait on the maturity level, you are teaching them how to take care of their teeth, brushing & flossing, etc.