This retrospective study aims to test the third molar maturity index (I3M) cut-off value of 0.08 for 18 years old in Dominican Republic population. Orthopantomograms of 513 subjects (284 females and 229 males) were evaluated, intra-and inter-observer agreement, ICC (intra-class correlation coefficient) values were 0.88% (95 % CI 0.86% to 0.91%), and 0.93% (95% CI 0.90% to 0.96%), for the intra-and inter-observer reliability, respectively. Accuracy in females was 0.96 (95% CI: 0.93-0.97); the sensitivity was 0.99 (95% CI: 0.96-0.99) and specificity was 0.92 (95% CI: 0.86-0.95). In males, the accuracy was 0.96 (95% CI: 0.93-0.98); the sensitivity was 0.94 (95% CI: 0.88-0.97) and specificity was 0.99 (95% CI: 0.95-0.99). The PPV (Positive Predictive Value) was 0.93 for females and 0.99 for males. The results of this study show that I3M can be used for discriminating adults from minors in Dominican Republic subjects around the legal age of 18 years old.