Despite policy and theoretical support for mixed-age classrooms in early childhood, research examining associations between age-mixing and children's outcomes is inconclusive and warrants further investigation, particularly in preschools serving children who are at risk for poor adjustment to formal schooling. One recent study conducted in preschool classrooms serving low-income children found negative associations between age-mixing and children's social and cognitive development. The current study extended this research by examining associations between classroom age composition (variability in ages of children in the classroom) and low-income preschool children's rates of change in school readiness. The sample consisted of 4417 preschool children enrolled in 207 classrooms in a large, diverse urban Head Start program. Multilevel modeling was employed to examine the main effect of classroom age composition, as well as the interaction between classroom age composition and children's age, as predictors of children's rates of change in emergent literacy, emergent numeracy, social and emotional skills, and approaches to learning. In contrast to previous research, classroom age composition was not associated with school readiness outcomes. This study contributes to the conflicting literature examining the associations between age mixing and children's school readiness and calls for a future research agenda to examine age mixing in context that is focused on sorting out these conflicting results. In the meantime, policymakers should consider other relevant factors when making decisions regarding mixed-age classrooms, such as family preference or the capability for teachers to individualize instruction to children based on their individual needs. (Contains 4 tables.)