My daughter's been going since she was a year old. She had some staining due to antibiotic use and I took her for regular cleanings. Most pediatric dentists will tell you they should start at age one, and a lot of pediatricians say by age two.
I can say that starting at such a young age was great for my daughter because she is now one of the best patients they have. She is very comfortable there and thinks it's actually fun to see the dentist. Her teeth are in great shape, too, which I think is more a factor of her diet and her dental hygiene (we're pretty vigilant about brushing, flossing, and using fluoride rinse), but anyway, the dentist said that by this age (she's 5 1/2) a lot of kids start showing up with cavities.
So, take your daughter soon. There are a lot of great books for preschoolers about going to the dentist, too - I'd try the Mister Rogers one if you can find it. I'd highly recommend a pediatric dentist if your insurance covers it - they just really cater to kids in a way that makes it fun. Ours has each room painted in a theme - underwater, jungle, etc., they let the kids wear sunglasses to shield the bright light, the dental hygienists are all young and perky, that sort of thing. Good luck!
