How early do most children seeing a pediatric dentist receive their first filling?

Your question makes two assumptions both of which are untrue: (1) most children need to see a pediatric dentist and (2) most children need treatment. Let's start at the beginning. The American Academy of Pediatric Dentistry recommends children receive their first exam before age 1. In reality, this more for the parent(s) than the child. The concern is to make sure the parent is not putting the baby to bed with a bottle. This causes carbohydrates (complex sugars) to lay on the teeth for hours. The result is a condition called "nursing bottle caries," a type of tooth decay that effects almost all the teeth in the mouth with cavities the girdle the tooth at the gumline. This children often have to be hospitalized for treatment. Fortunately, most (medical) pediatricians warn 1st-time mothers about this condition and it is much less common than in the past, even without a formal dental evaluation. Most general dentists advise parents to bring a child between 18-36 months for their first exam. Usually, there is no treatment required.