Baby teeth can start coming in as early as five months. By one year, a child typically has six baby teeth: usually the top four front teeth, and the bottom two front teeth. By three years old, children should have all 20 of their baby teeth.
Some children are actually born with teeth. We call them natal teeth, or milking teeth. In fact, in my residency I was called to the hospital to evaluate a 12-hour old child with multiple teeth. Because there was a risk of aspiration, we had to pull them. I really hope the Tooth Fairy made an early stop to that baby!