These are the stages of dental development that are important in the field of orthodontics. The ages given are approximate - the range of normal is very broad.
Dentition is the dentist word for "the teeth".
6-8 Months; Primary Dentition - Eruption of Baby Teeth
The first baby tooth is often a lower front tooth, erupting at about age 8 months. Gradually, the rest of the baby teeth come in and by age 27 months all of the baby teeth have erupted (10 top ones, 10 bottom ones). Baby teeth almost always look straight and cute.
6-7 Years; First Transition - Primary to Mixed Dentition
At around ages 6 to 7, many things start to happen. The front baby teeth get loose and begin to fall out. The permanent front teeth start to come in. Also, the 6 year molars start to erupt in the back corners of the upper and lower jaws. They do not replace baby teeth. They just show up on their own. By age 8 or so, most people have 4 permanent upper front teeth, 4 permanent lower front teeth, and all of the six year molars in the back corners, and the rest of the teeth are baby teeth.
7-8 up to 10-11 years. This stage of dental development is called the MIXED DENTITION. Any orthodontic issues are recognizable at this age. A few can and should be treated at this age. This is good time for an examination by an orthodontist.
10-13 Years; Second Transition - Mixed to Permanent Dentition
At around age 10, a few of the back baby teeth get loose and fall out, and are replaced by permanent teeth. This process continues over 18 to 24 months until all of the baby teeth are gone and are replaced by permanent teeth.
Finally, at about age 12, the twelve year molars erupt. They do not replace baby teeth-they just show up in the very back corners.
This is the classic "moment" for braces. All of the teeth are in, and the face is still growing, there is a large peer group sharing the same experience-the best time to finish a treatment.
17-20 Years and Beyond-Mature and Adapting
If wisdom teeth have formed (sometimes they do not) and if there is adequate room for them (sometimes there is not) they will erupt in the late teens. Facial growth is generally complete. Changes in the face and the bite are generally adaptations, not active growth. Orthodontic treatment still works, but there may be some limitations as to what can be accomplished.