DS got his appointment the other day, he's 16mo - think it depends on your dentist; as Theirmum says it's about getting them used to the place rather than having concerns about their teeth. As it is, I've changed his appointment and he'll be coming along with me to mine instead.
I have always taken my dd to the dentist with me, have let her be in the room each time when I have had my teeth checked and when i had work down to fit a crown.
This way she was happy to actually be seen by the dentisit as she knew mum wasn't scared or frightened and was used to the dentist, he wasn't a stranger and knew that he wouldn't hurt her even if his tools do make lots of noise.
I and all my colleagues did it from 2yrs. Just to let you know your child isn't officially registered until they have had their first appointment and you have signed a claim form that is dispatched, then they get on the nhs list.