It is generally recommended to start taking them to the dentist when their first tooth comes through or by 12 months, whichever comes first. Whilst no treatment may be performed then, this is a good strategy to get kids used to seeing the dentist. At that first appointment, our dentists will discuss strategies to maintain your child's oral hygiene and health in an easy and non-threatening way. The first time our young patients come in, they often sit in their parents' lap for a joy ride in the dental chair!
Here's some more info from the Australian Dental Association.