Although high accuracy rates have been achieved for adult skeletal sex estimation, it is unclear at what age sexually dimorphic traits of the skull become fully developed and useful in subadults. Four linear distances that capture sexual dimorphism—the nuchal crest, mastoid process, glabella, and mental eminence—were measured in a longitudinal sample of lateral cranial radiographs from 10 males and 10 females in the Denver Growth Study (total n = 190). The age full trait expression was attained was compared to that of dental maturity (i.e., third molar eruption). The mental eminence reached full expression significantly earlier than dental maturity (p < 0.001), although a high degree of variability was observed. Ages of expression of the other traits did not differ significantly from dental maturity. These results suggest that in forensic cases, where misclassification can have substantial impacts, cranial traits cannot yet be used reliably for sex estimation prior to third molar eruption.