This study examines how student age affects group assignments within classrooms and schools, asking whether teachers accurately stratify students based on true ability or use age as a proxy for ability. Using ordinary least squares analysis on a sample of English students, I find that older students are more likely to be placed in higher ability groups and academic streams. In Year 2, each additional month of age increases the chance of being in the top group by 2.3 percentage points and decreases the chance of being in the bottom group by 1.2 percentage point. In Year 6, the chances of being in the highest stream increase by 2.2 percentage points per month of age, while the likelihood of being in the lowest stream decreases by 0.8 percentage points. I also find that test scores mediate the effect of age on group placement, with this mediating effect decreasing as students age. These findings suggest that age causally influences academic group placement, potentially impacting long-term educational outcomes.