Gender differences in spatial aptitude are well established by adulthood, particularly when measured by tasks that require the mental rotation of objects (Linn & Petersen, 1985; Voyer, Voyer, & Bryden, 1995). Although the male advantage in mental rotation performance represents one of the most robust gender differences in adult cognition, the developmental trajectory of this male advantage remains a topic of considerable debate. To address this debate, we meta-analyzed 303 effect sizes pertaining to gender differences in mental rotation performance among 30,613 children and adolescents. We found significant developmental change in the magnitude of the gender difference: A small male advantage in mental rotation performance first emerged during childhood and then subsequently increased with age, reaching a moderate effect size during adolescence. Procedural factors, including task and stimulus characteristics, also accounted for variability in reported gender differences, even when controlling for the effect of age. These results demonstrate that both age and procedural characteristics moderate the magnitude of the gender difference in mental rotation throughout development. (PsycINFO Database Record (c) 2019 APA, all rights reserved).