There are several stages of teeth growth.
The teeth of a new-born baby grow inside the gums and jaws, though they are not visible. The baby starts teething at the age of about six months when the first tooth erupts through the gum.
In most children, by the age of three, all twenty of the deciduous teeth will have erupted. These teeth are sometimes called baby teeth or milk teeth.
The child’s jaw goes on growing while the deciduous teeth do not. If the deciduous teeth stayed in place they would be far too small for an adult. This is the reason why they are replaced by the larger permanent teeth. The change begins at about the age of six. The permanent teeth grow up under the deciduous teeth. The roots of the deciduous teeth dissolve and the crowns become loose. Finally the crowns drop out. Then the permanent teeth erupt in their place.
By the age of thirteen or fourteen, twenty-eight of the permanent teeth have normally erupted. The four remaining molars usually erupt between the ages of seventeen and twenty-one. These are called the wisdom teeth.