Research Findings: The classroom physical literacy environment is a malleable and domain-specific contributor to children's emergent literacy acquisition. However, what shapes the physical literacy environment remains unclear. In this study, we drew on a sample of 895 4-year-olds across 223 classrooms from the Professional Development Study and tested the role of classroom age composition in shaping the physical literacy environment. Additionally, we examined the extent to which the physical literacy environment explained the associations between classroom age composition and emergent literacy outcomes. We found that classrooms with a higher proportion of younger children (2- or 3-year-olds) provide less book use, writing materials, and writing around the room. Notably, the lack of writing displayed around the room served as one potential reason why 4-year-olds demonstrate smaller gains in expressive language skills in these classrooms. Practice or Policy: Together, these findings highlight the need for investing in the classroom physical literacy environment to ensure that it meets the needs of all children in mixed-age classrooms.