The age at which students enter school is increasing. More parents are delaying their child’s entry, and U.S. states are moving school entry cutoffs earlier, mainly because older students outperform younger ones on many educational outcomes. Much of the literature interprets advantages held by older students as benefits to entering school older, but because entering older means being older when students take tests, it is unknown if performance differences are attributable to entry age or test age. Policy and parent behavior depend on which age effect matters more. Using a natural experiment from the province of British Columbia, Canada, that temporarily altered entry dates, I estimate an upper bound of the test age effect and a lower bound of the entry age effect. Results show that the upper bound of the test age effect is much larger than the lower bound of the entry age effect.