Head trauma is the most frequent injury sustained by children in car crashes, and the neck plays a key role in governing head kinematics during the crash. Pediatric anthropomorphic test devices (ATDs) are used to assess the risk of head injury, yet the pediatric ATD neck is a size-scaled model of the adult ATD neck, with no consideration for the tissue properties and morphological changes during human development. To help understand the effects of maturation on the changes in neck flexion biomechanics, this study compared the passive cervical spine flexion of children to adults in specific age groups (6-8, 9-12, 20-29, 30-40 years). Subjects with restrained torsos and lower extremities were exposed to a 1g inertial load in the posterior-toanterior direction, such that the head-neck complex flexed when the subject relaxed their neck musculature. Surface electromyography with audio feedback was used to coach the subjects to relax their neck musculature. A multicamera 3-D target tracking system was employed to capture the motion of specific landmarks on the head (Frankfort Plane) and thoracic spine (T1 and T4). Neck flexion angle with muscles relaxed was calculated for each subject. Neck flexion angle significantly decreased with age, with changes in head-to-neck girth ratio partially explaining the decrease. A statistically significant increase in cervical spine flexion was found in adult females compared to adult males. Data also illustrate this trend in children, but it was not statistically significant. In summary, these results demonstrate an increased passive cervical spine flexion in children compared to adults, and females compared to males. These data will help guide the development and validation of pediatric ATDs.