Data from 1,407 preschoolers were used to examine the implications of classroom age composition for the early learning and development of 4-year-olds in classrooms with 3- and 5-year-olds also in attendance. Results suggest that a greater number of younger classmates did not detract from 4-year-olds’ language development, literacy performance, or inhibitory control, nor did having older peers consistently facilitate learning in these domains. However, 4-year-olds who entered school with low inhibitory control and print knowledge demonstrated greater gains in both domains when attending classrooms with more same-age or older classmates than when in classrooms with more younger peers. When taken together, these results suggest that classroom age composition, in prekindergarten programs serving mostly 4-year-olds, for the most part has little consequence except for those 4-year-olds entering school with lower skill levels in key domains, in which case having older peers is of benefit. (author abstract)