We estimate the effects of relative age in kindergarten using data from an experiment where children of the same age were randomly assigned to different kindergarten classmates. We exploit the resulting experimental variation in relative age in conjunction with variation in expected kindergarten entry age based on birth date to account for negative selection of some of the older school entrants. We find that, holding constant own age, having older classmates on average improves educational outcomes, increasing test scores up to eight years after kindergarten, and raising the probability of taking a college-entry exam. These findings suggest that delaying kindergarten entry, or so-called academic "red-shirting," does not harm other children--and may in fact benefit them--consistent with positive spillovers from higher-scoring or better-behaved peers.