Recent research has shown that the age composition of preschool classrooms influences children's early learning. Building on prior research, this study examines whether the association between classroom age composition and children's learning and development vary based on classroom quality and teacher characteristics using a subset of the Family and Child Experiences Survey (FACES), a nationally representative sample of 3- and 4-year-old children attending Head Start (n = 2,829). Results revealed that the association between age composition and children's academic skills was dependent on classroom quality and that classroom quality was less predictive of children's skills in mixed-age classrooms. Teacher education but not experience also moderated the influence of age composition such that mixed-age classrooms taught by a teacher with higher education were not associated with decreased literacy gains among older children.