Single-date school entry systems create large age differences between children in the same grade. Older students have been shown in the literature to outperform younger students along many elementary school outcomes, and some post-schooling outcomes. Little evidence exists about the size of these advantages in high school. Data from British Columbia, Canada are used to estimate the effect of age on test scores in grades 4, 7, and 10. I estimate that older students still have a sizable skill advantage in grade 10 across numeracy, reading, and writing tests. The advantage is strongest for girls and low-income students. The results suggest a certain degree of permanence to age related skill differences, which may spill over and affect adult outcomes.