The human mouth develops two distinct sets of teeth over a lifetime: the 20 primary (deciduous) teeth and the 32 permanent (adult) teeth. This transition begins around age six and continues into early adulthood. The primary teeth serve as guides for the permanent teeth. Their roots are gradually absorbed by the developing permanent teeth below, causing the primary teeth to loosen and shed, a process called exfoliation. This shedding clears the path for the permanent teeth to erupt and establish the final adult dentition.
The Replacement Map: Incisors and Canines
The most straightforward replacements occur in the front of the mouth, where the permanent teeth directly succeed their primary namesakes. The initial primary dentition includes eight incisors, which are the flat, sharp teeth used for cutting food. The four permanent central incisors are typically the first permanent teeth to erupt, appearing around six to eight years of age.
These are followed by the four permanent lateral incisors, which generally emerge between seven and nine years old. This phase of replacement is a direct one-for-one swap, with the permanent incisors moving into the exact space vacated by the primary incisors.
The four primary canines, the pointed teeth positioned at the corners of the mouth, are also replaced directly. The permanent canines erupt next, usually between 9 and 13 years of age, fitting into the same position as the primary canines. This set of 12 permanent teeth—eight incisors and four canines—simply replaces the 12 primary teeth of the same type.
The Transition: Primary Molars to Permanent Premolars
The posterior teeth undergo a more complex change, as the primary molars are not replaced by permanent teeth of the same name. The primary dentition includes eight molars, which are responsible for grinding food. These primary molars hold the space for a different type of permanent tooth: the premolar.
The eight primary molars are replaced by eight permanent premolars, often called bicuspids, which are entirely new tooth types not present in the primary set. Permanent premolars have characteristics of both the canines and the molars, serving a transitional function in chewing. They are positioned between the permanent canines and the permanent molars.
The permanent first premolars typically erupt between 10 and 12 years of age, replacing the primary first molars. The permanent second premolars follow, emerging between 10 and 12 years of age, taking the place of the primary second molars. This switch allows for the necessary space rearrangement that accommodates the full set of 32 permanent teeth.
Permanent Teeth That Do Not Replace Primary Teeth
A dozen permanent teeth, the molars, do not replace any primary teeth but erupt into new positions further back in the jaw. These teeth are sometimes called non-succedaneous teeth because they do not have a primary predecessor. The first permanent molars are often the first permanent teeth to erupt, appearing around age six, behind the primary second molars.
These four teeth are commonly known as the “six-year molars” and secure the jaw structure early in the transition phase. The four second permanent molars follow, erupting around 11 to 13 years of age, and are sometimes called the “twelve-year molars.”
Finally, the four third permanent molars, or wisdom teeth, typically emerge between the ages of 17 and 21, if they develop at all. The addition of these 12 molars to the 20 replacement teeth explains the difference in count between the 20 primary teeth and the full 32 permanent teeth.