You'll get many different responses/opinions on this one. Most of my friends say 3 years old.
My kids are 3 and 5 and have been going since their first few teeth sprouted. I did this for a few reasons. I've never feared the dentist and wanted my kids to have the same experience if possible. I'd hate for them to be 3, never seen a dentist and all of a sudden need some dental work. I thought it would be better for them to have a dentist they know and feel comfortable around should that day come that they need something done. So far we've only had check ups. Another reason we started so young was because it's covered under our insurance. We see a pediatric dentist and the kids didn't sit in the chair until they were ready to do so, my son still sits in my lap and he just turned 3. So, most likely, they will just have your child at that age sit on your lap. Don't assume the dentist is a scary place for a child, it doesn't have to be. All the dentist does on check ups is look inside the mouth, make sure everything is OK and then cleans the teeth which only takes moments but is nice because he/she can get the plaque that just doesn't come off even with daily brushing.
If you aren't already, make sure you are brushing your childs teeth twice a day, even if he only has four teeth. The sooner they get used to having their teeth brushed the easier it is in the long run.
Good luck,
M.