The permanent teeth, also known as the secondary dentition, develop and emerge over a period spanning more than a decade. This growth is complex, involving two distinct phases: the visible eruption of the crown into the mouth and the subsequent, hidden development of the root structure. Understanding this timeline is fundamental for monitoring dental health, as the completion of root formation marks the biological end of the tooth’s development. This developmental sequence determines the tooth’s long-term strength and vitality.
The Timeline for Permanent Tooth Eruption
The visible emergence of permanent teeth typically begins around age six with the eruption of the first molars, often called the “six-year molars,” and the lower central incisors. This stage, where the crown breaks through the gums, continues gradually over the next several years. The incisors generally appear between the ages of six and nine.
Following the incisors, the premolars—which replace the primary molars—and the canines erupt, marking the end of the mixed dentition phase. Most of the 28 permanent teeth, excluding the third molars, are typically visible in the mouth by the age of 12 or 13. The second molars usually complete their eruption around this time, between 11 and 13 years old.
The third molars, or wisdom teeth, are the final teeth to erupt, generally between 17 and 21 years of age. Although the crown is fully visible, the tooth’s physical growth is not complete.
Defining the End of Growth: Root Apex Closure
The precise biological point at which a permanent tooth stops growing is defined by the completion of its root, a process called root apex closure. Eruption occurs when only about two-thirds of the root is formed, leaving a wide, open hole at the root tip, known as the open apex. The remaining root structure continues to develop for years after the tooth has appeared in the mouth.
The root apex gradually narrows as more dentin is deposited along the walls of the root canal, eventually forming a constricted opening. This closure typically happens approximately two to three years following the tooth’s eruption. For most permanent teeth, root completion occurs between the ages of 9 and 16, with the canines and second premolars often finishing later than the incisors.
The distinction between an open and closed apex is significant for the tooth’s health and strength. An open apex means the dental pulp, which contains the nerves and blood vessels, can more easily receive nutrients and repair itself if injured. Once the apex is closed, the tooth is considered fully mature, having reached its maximum physical size and structural development. Wisdom teeth mature later, around 19 to 23 years old.
Post-Growth Life of Permanent Teeth
Even after the root apex is fully closed, the tooth is not a static structure; it undergoes continuous internal and external modifications throughout life. The internal pulp chamber, which houses the living tissue, steadily reduces in size due to the ongoing deposition of secondary dentin. Odontoblast cells, which are responsible for creating the tooth’s dentin, continue to produce this material slowly after the tooth’s full formation.
This slow, continuous deposition of secondary dentin is a natural aging process that acts as a protective measure, strengthening the tooth by making the pulp chamber smaller. External physical forces also affect the tooth structure, including attrition (wear from tooth-to-tooth contact during chewing and grinding) and erosion (loss of tooth structure due to acid).
While the size and shape of the tooth’s outer structure are set once the root is complete, these internal and external changes demonstrate that the tooth is actively maintained throughout its lifespan. The continuous activity of the pulp tissue, even in a mature tooth, allows it to respond to environmental stresses and mechanical loading.