When babies are born, they already have most of their teeth under their gums. The first tooth usually begins to erupt by the age of six months, although the exact age can vary from one baby to another. The first two teeth to come in are usually in the bottom middle, followed by the four in the upper middle. Most children have a complete set of 20 baby teeth by the time they turn 3. Some children do not get their teeth at the same time as their peers. This can be caused by several factors. If a child does not have any teeth by the age of 18 months, he or she should be taken to a pediatric dentist for an evaluation.