Abstract: Objective: To study the influence of age on extraction of mandibular third molars by cone beam computed tomography, and evaluate the risk of early removal of mandibular third molar. Methods: One hundred and fifty mandibular third molars which met the inclusion criteria were selected from patients in our hospital between June to December 2015, including 47 teeth in 16-18 age group, 60 teeth in 19-24 age group, and 43 teeth in 24 age above group. The mandibular height (a), the ratio of the residuary mandibular height (b) to mandibular height (a), and the shortest distance between teeth to mandibular canal were measured. The differences between different age groups were analyzed. Results: There was no significant difference in the mandibular height between three age groups. Among three groups, the ratio of b/a of 24 age above group was the largest, followed by 19-24 age group (P<0.05). In terms of the shortest distance between teeth to mandibular canal, the 19-24 age group and 24 age above group were larger than 16-18 age group, and there was no significant difference between 19-24 age group and 24 age above group. However, there was no significant difference in the rate of intimate contact between the teeth and the mandibular canal in three age groups. Conclusion: Removal of mandibular third molars in the 19-24 age group may be the most appropriate and least risky.