BACKGROUND: Researchers have begun to focus on age and sex differences in concussion outcomes. Results suggest that younger athletes and female athletes may take longer to recover from a concussion. However, little is known about the interactive effects of age and sex on symptoms, neurocognitive testing (NCT), and postural stability. HYPOTHESIS/PURPOSE: The purpose of the study was to examine sex and age differences in symptoms, NCT, and postural stability following concussion. We hypothesized that high school and female athletes would have worse symptoms, NCT, and postural stability than college and male athletes, respectively. STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: A total of 296 concussed athletes from a multistate, 2-year study were enrolled in this study. Participants completed the Immediate Post-Concussion Assessment and Cognitive Test (ImPACT) and Post-Concussion Symptom Scale (PCSS) at baseline and again at 2, 7, and 14 days after concussion. Participants completed the Balance Error Scoring System (BESS) at 1, 2, and 3 days after concussion. RESULTS: Female athletes performed worse than male athletes on visual memory (mean, 65.1% and 70.1%, respectively; P = .049) and reported more symptoms (mean, 14.4 and 10.1, respectively) after concussion (P = .035). High school athletes performed worse than college athletes on verbal (mean, 78.8% and 82.7%, respectively; P = .001) and visual (mean, 65.8% and 69.4%, respectively; P = .01) memory. High school athletes were still impaired on verbal memory 7 days after concussion compared with collegiate athletes (P = .001). High school male athletes scored worse on the BESS than college male athletes (mean, 18.8 and 13.0, respectively; P = .001). College female athletes scored worse on the BESS than high school female athletes (mean, 21.1 and 16.9, respectively; P = .001). CONCLUSION: The results of the current study supported age differences in memory and sex differences in memory and symptoms and an interaction between age and sex on postural stability after concussion that warrant consideration from clinicians and researchers when interpreting symptoms, specific components of NCT, and postural stability tests. Future research should develop and assess interventions tailored to age and sex differences and include younger (<14 years) participants.