This study examines the age at which a femoral physeal failure ceased to occur in a mouse model of medial collateral ligament (MCL) testing. Biomechanical testing of the MCL with load to failure can result in physeal failure rather than MCL failure in skeletally immature animals. Failure mode depended significantly on age (p<0.05). Sixty percent of the knees tested at 4 months failed at the physis rather than at the ligament, whereas, only ten percent of the knees tested at 5 and 6 months failed at the physis. The mean ultimate force to failure for the specimens in which the failure occurred at the ligament was 8.1N with a higher values for the right side versus the left (p<0.05). For the specimens in which the failure occurred at the physis, the mean ultimate force to failure was 11.2N. We now consider that 5 month old mice are functionally skeletally mature and old enough to be tested biomechanically with few failures at the physis.