Youth football players ages 8-12 may incur hundreds of repeated head impacts (RHI) each season. Evidence suggests concussive brain injury during childhood may disrupt normal developmental processes resulting in long-term impairments. However, little research has investigated the long-term effects of incurring RHI during critical periods of neurodevelopment. Rapid myelination and cerebral blood flow rates, peaks in regional cortical thickness and volumes of specific structures, refinement of regional connectivity, and other neurodevelopmental changes occurring in the brain from ages 10-12 could create a window of vulnerability to RHI. The objective of this research was to determine the relationship between exposure to RHI prior to age 12, during a critical period of neurodevelopment, and later-life brain structure and function. Former National Football League (NFL) players ages 40-65 were divided into two groups based on their age of first exposure (AFE) to RHI through tackle football: AFE <12 and AFE ≥12. In the first study, we observed significantly lower scores on objective tests of executive functioning, memory, and estimated verbal IQ in those who began playing football prior to age 12 compared to those who began playing at age 12 or older. Next, we used diffusion tensor imaging (DTI) to examine the structural integrity of the corpus callosum (CC) and observed that the AFE <12 group had significantly lower fractional anisotropy (FA) as well as a greater decline in FA with age in anterior CC regions than the AFE ≥12 group. Lastly, we used advanced DTI tractography techniques to examine seven CC regions. Significant differences between AFE groups in associations between CC diffusion measures and cognition, mood, and behavior were found. The results of this research suggest that incurring RHI through tackle football during a critical neurodevelopmental period prior to age 12 may result in later-life structural and functional consequences, including cognitive, mood, and behavioral impairments; alterations in white matter structure; and greater vulnerability of white matter to the normal aging process. If replicated with longitudinal designs, larger samples, and athletes whose highest level of play was youth, high school, or college, these findings may have implications for safety recommendations for youth sports.