Due to increasingly numerous international migrations, estimating the age of unaccompanied minors is becoming of enormous significance for forensic professionals who are required to deliver expert opinions. The third molar tooth is one of the few anatomical sites available for estimating the age of individuals in late adolescence. This study verifies the accuracy of Cameriere's cut-off value of the third molar index (I3M) in assessing 18 years of age. For this purpose, a sample of orthopantomographs (OPTs) of 397 living subjects aged between 13 and 22 years (192 female and 205 male) was analyzed. Age distribution gradually decreases as I3M increases in both males and females. The results show that the sensitivity of the test was 86.6%, with a 95% confidence interval of (80.8%, 91.1%), and its specificity was 95.7%, with a 95% confidence interval of (92.1%, 98%). The proportion of correctly classified individuals was 91.4%. Estimated post-test probability, p was 95.6%, with a 95% confidence interval of (92%, 98%). Hence, the probability that a subject positive on the test (i.e., I3M < 0.08) was 18 years of age or older was 95.6%.