This paper discusses a method for assessing adult age based on the relationship between age and measurement of the pulp/tooth area ratio (RA) of the second molar teeth. The sample consisted of 312 Italian white Caucasian patients (135 men, 177 women) aged between 14 and 24 years. Orthopantomograph samples (OPGs) were analyzed by the same observer and calibrated by kappa statistics. Probabilities of an individual being older than 18 years of age (adult age) were derived using measurements of the ratio pulp/tooth area (RA) of the second molar teeth and the dichotomized stages of third molar development (Tm) as explanatory variables. These results were exploited to establish a threshold value to assign an individual to juvenile or adult age. A cut-off value of RA* = 0.088 was applied if Tm = 0 and RA* = 0.097 if Tm = 1. The sensitivity of this test was 91% and its specificity 94.5%. The proportion of individuals with correct classifications was 92%.