Context: Research has shown age and sex differences in concussion outcomes. However, little is known about the interactive effects of age and sex on symptoms, neurocognitive testing (NCT), and postural stability. Objective: The purpose of the study was to examine sex and age differences in symptoms, NCT, and postural stability following concussion. We hypothesized that males and younger athletes would have worse symptoms, NCT, and postural. Design: Prospective cohort study Setting: This study was performed in a controlled laboratory setting. Patients or Other Participants: A total of 222 concussed athletes from a multi-state, two-year study volunteered to participate and were classified as college males (n= 37,age =19.5+1.08 years, mass =93.8+15.6 kg, height =183.8+7.24 cm), college females (n =35 , age =18.9+1.55 years, mass =66.7+9.1 kg, height =170.3+8.22 cm) high school males (n=121 , age =15.5+1.19 years, mass =76.6+15.5 kg, height =176.6 +7.59 cm), high school females (n= 31,age =15.4+1.22 years, mass =63.9+10.4 kg, height =164.4+7.13 cm). Interventions: The independent variables in this study were time (baseline, 2, 7, and 14 days post-concussion), sex (male, female), and age (high school, college). Partici-pants completed the Immediate Post-concussion Assessment and Cognitive Test (ImPACT) and Post-concussion Symptom Scale (PCSS) at baseline; and at 2, 7, and 14 days post-concussion. Participants completed the Balance Error Scoring System (BESS) at 1, 2, and 3 days postconcussion. Main Outcome Measures: Dependent variables included ImPACT composite scores (verbal/ visual memory, reaction time, processing speed), PCSS scores, and BESS scores. A 4 (time) x 2 (sex) x 2 (age) repeated measures ANOVA was performed to compare group differences in concussion symptoms and ImPACT composite scores and another 3 (time) x 2 (sex) x 2 (age) ANOVA was performed to compare group differences in BESS scores. Post-hoc comparisons were performed using Tukey’s HSD, and a priori statistical significance was set at = 0.05, and the Bonferroni was applied for multiple comparisons. Results: Females performed worse than males on visual memory (M=65.1+15.7: 70.1+15.4, F(1,118), 3.95, P=.049, η2= .032) and reported more symptoms (M=14.4+6.7: 10.1+ 10.3) High school athletes performed worse than college athletes on verbal (M= 78.8+12.5: 82.7+13.7, F(3,116), 8.03, P= .001, η2= .171) and visual (M=65.8+16.1: 69.4+15.6, F(3,116), 3.64, P=.02, η2= .09) memory. High school males scored worse on the BESS than college males (M= 18.8 +8.4: 13.0+6.8, F(1,111), 7.29, P=.008, η2= .061). College females scored worse on the BESS than high school females (M=21.1+6.9: 16.9+8.7, P=.008). Conclusions: The findings from the current study suggest that concussed high school and collegiate athletes exhibit different outcomes following concussion. Specifically, age and sex should be considered when interpreting symptoms, NCT, and postural instability following concussion.