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Objective Typically, studies on outcomes after traumatic brain injury (TBI) have investigated whether a younger age at injury is associated with poorer recovery by comparing 2 age groups rather than participants injured across childhood. This study extended previous research by examining whether the influence of age on recovery fits an early vulnerability or critical developmental periods model. Methods Children with a TBI (n = 181) were categorized into 4 age-at-injury groups—infant, preschool, middle childhood, and late childhood—and were evaluated at least 2-years post-TBI on IQ. Results Overall, the middle childhood group had lower IQ scores across all domains. Infant and preschool groups performed below the late childhood group on nonverbal and processing speed domains. Conclusions Contrary to expectations, children injured in middle childhood demonstrated the poorest outcomes; this age potentially coincides with a critical period of brain and cognitive development.