Having children of multiple ages in the same preschool classroom is a common practice, and age composition has been shown to shape children's learning. However, there is little understanding of the mechanisms that link age composition to children's development. In this study, we examined the extent to which classroom age composition shaped children's classroom engagement, as well as the mediating role of classroom engagement in the links between classroom age composition and children's language and literacy gains. The data were drawn from the Teacher Professional Development Study, a study of 895 4-year-olds across 223 classrooms. We found that positive engagement with teachers mediated associations between classroom age composition and children's vocabulary gains. Specifically, 4-year-olds in classrooms with a greater number of younger classmates experienced less positive engagement with their teachers, which in turn, contributed to smaller vocabulary gains across the year. In addition, being with a greater number of older classmates was associated with lower negative engagement in the classroom. Results are discussed in relation to exploring the critical role of children's individual-level classroom experiences in mixed-age preschool classrooms. (author abstract)