Purpose: The anatomic origin for prepubertal vowel acoustic differences between male and female subjects remains unknown. The purpose of this study is to examine developmental sex differences in vocal tract (VT) length and its oral and pharyngeal portions. Method: Nine VT variables were measured from 605 imaging studies (magnetic resonance imaging and computed tomography) of subjects between birth and age 19 years. Given sex differences in growth rate (Vorperian et al., 2009), assessment of sex differences was done through use of a localized comparison window of 60 months. Analysis entailed applying this comparison window first to 4 discrete age cohorts, followed by a progressive assessment in which this comparison window was moved in 1-month increments from birth across all ages. Results: Findings document significant postpubertal sex differences in both the oral and pharyngeal portions of the VT. They also document periods of significant prepubertal sex differences in the oral region first, followed by segments in the pharyngeal region. Conclusions: Assessment of developmental sex differences using localized age ranges is effective in unveiling sex differences that growth rate differences may conceal. Findings on the presence of prepubertal sex differences in the oral region of the VT may clarify, in part, the anatomic basis of documented prepubertal acoustic differences.