Accurate pediatric thoracic force and deflection data are critical to develop biofidelic pediatric anthropomorphic test devices (ATDs) used in designing motor vehicle safety systems for child occupants. Typically, post-mortem human subject (PMHS) experiments are conducted to gather such data. However, there are few pediatric PMHS available for impact research, therefore novel methods are required to determine pediatric biomechanical data from children. In this study, we have leveraged the application of chest compressions provided in the clinical environment during pediatric cardiopulmonary resuscitation (CPR) to collect this fundamental data. The maximum deflection of the chest during CPR is in the range of chest deflections in PMHS impact experiments and therefore CPR exercises the chest in ways that are meaningful for biofidelity assessment. Thus, the goal of this study was to measure the force-deflection characteristics of the thorax of children and young adults during CPR. To do so, a force and deflection sensor was integrated into a patient monitor-defibrillator used during CPR in the Pediatric Intensive Care Unit and Emergency Department of a children's hospital. The sensor was interposed between the chest of the patient and hands of the rescuer during CPR compressions. Following a CPR event, thoracic force and deflection data were downloaded from the monitor-defibrillator for analysis. Each compression cycle was fit to a parallel spring-damper model, wherein stiffness and damping were linearly dependent on chest deflection. Average maximum chest deflection, force at maximum deflection, linear stiffness, and elastic and viscous model forces are reported for each subject and correlated with age. Eighteen subjects (11 females) ages 8 to 22 years were enrolled in the study and each received a mean of 2000 (Standard Deviation 2339) chest compressions during CPR. Average maximum chest deflection and corresponding force were 39 +/- 5 mm and 309 +/- 55 N respectively. When combined with our previous study of adult CPR data, and other data from the literature, our findings suggest that the stiffness of the thorax increases from youth to middle age, and then decreases in the elderly. CPR has the potential to provide data from a wide range of human subjects with which to study the effect of age on mechanics of thoracic deformation. Future studies will expand the sample size and age range of data collected to further explore the age-related changes in thoracic mechanics.