We present evidence of a positive relationship between school starting age and children's cognitive development from ages 6 to 18 using a fuzzy regression discontinuity design and large-scale population-level birth and school data from the state of Florida. We estimate effects of being old for grade (being born in September vs. August) that are…

California's Kindergarten Readiness Act of 2010 revised the cutoff date by which children must turn five for kindergarten entry in that year and established transitional kindergarten (TK), defined as the first year of a two-year kindergarten program for all students affected by the birthdate eligibility change. Since it has been implemented across…