Introduction:

Third molar maturity index (I3M) can be used as a method to provide probability of legal age majority; however, the method’s performance is known to be population-specific.

Objectives:

To assess the performance of I3M in Indonesian juvenile population using a new cut-off value calculation to determine whether an individual has reached age of 18 years.

Material and methods:

The sample consisted of 900 panoramic radiographs, with an even distribution of age and sex, from 15 to 23.98 years old. I_3M measurement was calculated by the sum of lengths from inner margins of the tooth apical ends, divided by the tooth length from apical to the end of the highest point of the crown. If apices were closed, the I_3M was counted as 0. Probability of an individual being above or below 18 years old given a certain I_3M was done using logistic regression, and optimal cut-off value was calculated with highest Youden’s index value.

Results:

The utilization of non-sex-specific cut-off value provided an accuracy of 0.88 for both sexes, with a recom­mended cut-off value of I_3M ≤ 0.1. The probabilities of an individual to be above 18 years old were 0.82 and 0.95 if I_3M ≤ 0.04 for males and females, respectively.

Conclusions:

The third molar maturity index can be an effective tool for accurately classifying whether an Indo­nesian’ individual is above or below 18 years old, with a recommended cut-off value of 0.1 for both sexes.