In many countries, assessment of legal age, also known as age of majority, has become increasingly important over the years. In China, individuals older than 18 years of age have full capacity regarding civil conduct and can be tried as an adult for criminal charges. Therefore, from a legal point of view, it is crucial to determine whether an individual is an adult. The developmental degree of the third molar is widely recognized as a suitable site for age estimation in late adolescence. This article uses the third molar maturity index (I3M) with a cutoff value of I3M = 0.08, which was established by Cameriere et al. in 2008, to distinguish whether an individual is a minor or an adult (≥ 18 years of age) in a northern Chinese population. A total of 840 digital orthopantomograms (OPTs) from 420 male and 420 female northern Chinese subjects aged 12 to 25 years were evaluated. It was found that an increase in I3M corresponded to a decrease in chronological age. In our study, I3M = 0.10 showed better accuracy in age discrimination in both men and women. This threshold also resulted in high sensitivity (0.929 and 0.809) and specificity (0.940 and 0.973) in males and females, respectively. The proportion of correctly classified subjects was 0.917 (95% CI, 0.898 to 0.935) in total, 0.938 (95% CI, 0.915 to 0.961) in male and 0.895 (95% CI, 0.866 to 0.925) in female subjects. Bayes post-test probabilities were 0.967 (95% CI, 0.947 to 0.986) in males and 0.983 (95% CI, 0.966 to 0.998) in females. These differences in threshold values between Chinese and Caucasian populations might be because the development of third molars is delayed in the Chinese population compared to the Caucasian population. In conclusion, I3M might be a useful method in legal and forensic practices to determine ages in late adolescence in northern Chinese individuals. However, a specific population should be tested before I3M is used for legal age estimation.
Keywords: Age estimation; Forensic odontology; Legal adulthood; Northern Chinese; Third molar maturity index.