Radiographs depicting third molars (M3s) have been used to estimate chronological age in juvenile and adult suspects, but accuracy of the method has been in question. This study provides age benchmarks for American whites (age range: 14 to 24 years) based on cases (n = 823) drawn from diplomates of the American Board of Forensic Odontologists in the United States and Canada. Maxillary M3 formation was slightly advanced over mandibular M3s, and root formation occurred earlier in males than females. Mean and median ages for M3 formation are tabled using Demirjian's eight-grade classification. Regression formulas and empirical probabilities are provided relative to the medicolegal question of whether an individual is at least 18 years of age. The M3 is the most variable tooth in the dentition, but situations arise where M3 formation is the only usable datum for age estimation.