Much of the literature estimating disproportionality in special education identification rates has focused on socioeconomic status, race, and gender. However, recent evidence suggests that a student's school starting age also impacts the likelihood they receive special education services, particularly in the early grades. I build on the evidence that the youngest students in a grade more likely to be diagnosed with Attention Deficit/Hyperactivity Disorder and more likely to be placed in special education by estimating the effect of school starting age on special education identification in Michigan. I also estimate heterogeneity in this effect by student characteristics and across school districts. Using a regression discontinuity design exploiting variation in kindergarten starting age generated by a statewide kindergarten entrance age policy, I find that the youngest students in a kindergarten cohort are 40% more likely (3.3 percentage points, p<0.001) to be placed in special education than are the oldest students, and that this effect persists through eighth grade. Despite little evidence of heterogeneity by gender, race, or socioeconomic status, I find some suggestive evidence that the effect is particularly large for white boys in the early elementary grades and for black girls in the later elementary grades. I find no evidence that these effects vary across school districts. Finally, I find exploratory evidence of variation by school cohort age composition, suggesting these effects are driven moreso by relative age comparisons than absolute age developmental differences. Given the importance of special education services to the academic success of children with disabilities, these findings have implications for schools and for policymakers seeking to improve special education program provision.