When Should Children Start School?

This paper studies causal effects informative for deciding the age when children should start kindergarten. I present evidence from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K) that standard instrumental variable strategies do not identify effects of delaying kindergarten entry for any subpopulation of interest. I propose and implement a new strategy for identifying individual-level education production function parameters. Estimates indicate that there can be decreasing and even negative returns to relative age: For the oldest children in a cohort, educational achievement in third grade decreases as their age relative to that of their classmates increases.