Using data from the Family and Child Experiences Survey 2009 Cohort (n = 1,073), this study considered the implications of mixed-age education for young children's academic achievement when they experienced continuity and/or changes in classroom age composition across two years in Head Start (at age 3 and age 4). Results from these analyses revealed that children in classrooms with a greater number of younger children during their second year in Head Start exhibited fewer gains in mathematics and language and literacy. Additionally, children who transitioned from being in classrooms with largely older classmates during year one to classrooms with largely same-age peers during year two exhibited greater gains in academics than children who experienced two years of mixed-age classrooms. Stability in children's teachers, one of the hallmarks of mixed-age programming, was not associated with children's academic achievement nor did it attenuate the negative consequences of mixed-age classrooms. [This is the in press version of an article published in "Journal of Applied Developmental Psychology."]