The federal Head Start program, designed to improve the school readiness of children from low-income families, often serves 3- and 4-year-old children in the same classrooms. Given the developmental differences between 3 and 4 year olds, it is unknown if educating them together in the same classrooms benefits one, both, or neither. Using data from the Family and Child Experiences Survey 2009 cohort, this study leveraged a peer effects framework to examine the associations between mixed-age classrooms and the school readiness of a nationally representative sample of newly enrolled 3- (n= 1,644) and 4-year-old (n= 1,185) Head Start children. Results revealed that 4-year-old children displayed fewer gains in academic skills during the preschool year when they were enrolled in classrooms with a greater number of 3 year olds; effect sizes corresponded to four-to-five months of academic development. In contrast, classroom age composition was not consistently associated with 3-year-old's school readiness.