According to the American Dental Association:

"Teeth vary in size, shape and their location in the jaws. These differences enable teeth to work together to help you chew, speak and smile. They also help give your face its shape and form.

At birth people usually have 20 primary (baby) teeth, which often erupt as early as six months of age. They are then shed at various times throughout childhood. By age 21, all 32 of the permanent teeth have usually erupted."