The objective of this research was to delineate age-based differences in specific thoracic injury diagnoses for seat belt restrained rear seat occupants and describe the associated injury causation in order to provide insight into how the load of the seat belt is transferred to occupants of various sizes. Using data from the Crash Investigation Research and Engineering Network (CIREN), 20 cases of rear seated, lap and shoulder belt restrained occupants with AIS2+ thoracic injuries in frontal crashes were reviewed. Seven were children and adolescents age 8-15 years, 5 were 16-24 years, 3 were 25-54 years, and 5 were 55+ years. Six of the seven 8-15 year olds sustained injuries to the lung in the form of pulmonary contusion or pneumothorax. Only three of the seven sustained a skeletal (sternum or rib) fracture;​ only one of these three involved multiple ribs bilaterally. In contrast, four of the five 16-24 year olds sustained at least one rib fracture - often multiple and bilateral. The adult cohort (25+ years) was involved in predominantly more minor crashes;​ however they all sustained complex rib fractures – seven of the eight involved multiple ribs, four of the eight were also bilateral. Belt compression – either from the shoulder belt or the lap belt – was identified as the primary cause of the thoracic injuries. Often, there was clear evidence of the location of belt loading from AIS 1 chest contusions or abrasions. These findings have implications for age-based thoracic injury criteria suggesting that that different metrics may be needed for different age groups.