After that first tooth breaks through, more and more teeth begin to appear. Most kids have their first set of teeth by the time they are 3 years old. These are called the primary teeth, baby teeth, or milk teeth and there are 20 in all. When a child gets to age 5 or 6, these teeth start falling out, one by one. A primary tooth falls out because it is being pushed out of the way by the permanent tooth that is behind it.