Like many other mammals, humans have two sets of teeth. "Baby" teeth (also called milk or deciduous teeth) start coming in at about 6 months, beginning with the central incisors in the mandible (lower jaw). Each tooth type (incisors, canines, premolars, molars) erupts on a predictable schedule. Permanent teeth begin to replace deciduous teeth at about 6 years and finish erupting by about 21 years of age.

X-rays provide views of the unerupted permanent teeth and tooth roots still within the bone. The crown of a tooth forms first, followed by the root. Scientists estimate age by comparing the stage of tooth formation in the X-rays and bone with know dental growth standards.

Example: Dental development at about 5 years.

x-rays of dental development in a 5 year-old

Images courtesy of: Smithsonian National Museum of Natural History

 

This child's deciduous teeth have all erupted, and the first permanent molars were beginning to emerge. The remaining permanent teeth were forming inside the bone.