There is growing evidence that school starting age impacts children's likelihood of receiving special education services, but less is known about variations in this effect. Using a regression discontinuity design, I found that the youngest students in a kindergarten cohort are 40% more likely (p < 0.001) to be placed in special education than are the oldest students, and that this effect persists through eighth grade. I found little evidence of heterogeneity by gender, race, or socioeconomic status, but some suggestive evidence that the effect is particularly great for White boys in early grades and for Black girls in the later elementary grades. I also found exploratory evidence that this effect is largest in schools with kindergarten cohorts that vary widely in age. These findings add new evidence to support policies to reduce age-induced bias in early special education placements, particularly those that address variation in this effect across students and schools. [For the corresponding grantee submission, see ED621422.]