Assessment of legal age, also known as age of majority is a controversial issue as there are few body biomarkers or evidence during late adolescence differentiating a subject from being a minor or adult. The third molar was recognized as a suitable site for age examination in late adolescence. We analyzed the development of the left mandibular third molar by the third molar maturity index (I3M) and a specific cut-off value of I3M =0.08, established by Cameriere et al. in 2008 and used it for discriminating between minors and adult black Africans from Gaborone, Botswana. A final sample of panoramic radiographs (OPTs) of 1294 living (582 males and 712 females) aged between 13 and 23 years, was evaluated. The real age decreased as I3M gradually increased. There was no statistically significant difference in the third molar development evaluated using I3M between males and females (p >0.05) across different I3M classes. Results of 22 contingency tables for different cut-off values indicated that I3M =0.08 was useful in discriminating between adults and minors. Precisely, for I3M =0.08, the values of accuracy or overall fraction of correctly classified were 0.91 in males with a 95% confidence interval (95% CI) 0.88 to 0.93 and 0.92 (95% CI, 0.90 to 0.93) in females. Values of sensitivity of the test or the proportion of participants being 18 years and older, were 0.88 (95%CI, 0.87 to 0.90) in males and 0.88 (95% CI, 0.90 to 0.93) in females, while values of specificity or proportion of individuals younger than 18 who have I3M <0.08, were 0.94 (95% CI, 0.91 to 0.96) in males and 0.96 (95% CI, 0.94 to 0.98) in females. Positive predictive values of the test, that the participants whose I3M <0.08 were adults, were 0.94 (95% CI 0.91 to 0.96) in males and 0.97 (95% CI, 0.94 to 0.98) in females while negative predictive values of the test, that the participants whose I3M was ≥0.08 were minors, were 0.88 (95% CI 0.85 to 0.90) in males and 0.97 (95% CI, 0.94 to 0.98) in females. The likelihood ratios of the positive test (LR+) were 13.67 (95% CI, 9.21 to 21.02) in males and 23.73 (95% CI, 14.20 to 42.28) in females while likelihood ratios of the negative test (LR-) were 0.12 (95% CI 0.10 to 0.16) in males and 0.12 (95% CI, 0.11 to 0.15) in females. Bayes post-test probabilities, p, were 0.94 (95% CI 0.90 to 0.98) in males and 0.97 (95%CI, 0.93 to 1.00) in females. These results indicate with high accuracy that I3M may be a useful alternative method in legal and forensic practice to discriminate individuals of black African origin who are around the legal adult age of 18 years in Botswana. Further studies should address the usefulness of this method and specific cut-off for different adolescent populations.