Classroom Age Composition and the School Readiness of 3- and 4-Year-Olds in the Head Start Program.

The federal Head Start program, designed to improve the school readiness of children from low-income families, often serves 3- and 4-year-olds in the same classrooms. Given the developmental differences between 3- and 4-year-olds, it is unknown whether educating them together in the same classrooms benefits one group, both, or neither. Using data from the Family and Child Experiences Survey 2009 cohort, this study used 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-year-olds (n = 1,644) and 4-year-olds (n = 1,185) in the Head Start program. Results revealed that 4-year-olds displayed fewer gains in academic skills during the preschool year when they were enrolled in classrooms with more 3-year-olds; effect sizes corresponded to 4 to 5 months of academic development. In contrast, classroom age composition was not consistently associated with 3-year-olds' school readiness.