I started taking my kids (I have 3 now) when they were 2 years old. It was a scaled back visit, but it got them comfortable with going. When my youngest was almost 3 he was so comfortable with going he sat by himself in the chair and even had his teeth cleaned!
The advances in dental care have changed so much since I was young that we are already going in for a consultation for my daughter's teeth with an orthodontist. At 7 they can apparently do things now to possibly avoid full braces down the road. I had to wait until I was in highschool to get mine! Not to mention, schools now require a dental checkup.
While I thought it was early to start taking my kids, I am glad that I did because some of my friends have a hard time even getting their kids into the chair. Mine love it! Start slow and work up to it - I had to sit in the chair with them at least once that I remember, but well worth it in the long run..Good luck.