Performance in reasoning tasks such as Raven's matrices experiences a dramatic increase over cognitive development, but the mechanisms responsible for this increase are unknown. Many cognitive processes are involved in a matrix task and could potentially change with age; strategy use appears to be a good candidate, as it typically improves over development and has a large impact on reasoning performance in adults. The present study tested the role of effective strategy use in Raven's standard progressive matrices in groups of 6-, 8-, 10-, 12-, 14-, 16-, and 18-year-olds (total N = 474). Strategy use was assessed with behavioral measures of gaze patterns in Raven's matrices. We also measured working memory capacity (WMC), a good predictor of strategy use in adults, using a battery of complex spans. The results showed that the effective strategy of constructive matching substantially increased with age, along with performance. Strategy use mediated over half the effect of age on reasoning performance. Older participants were also better at adapting strategy use to difficulty of the problems. Effective strategy use was beneficial to the same extent for participants of all ages. Age-related improvements in strategy use occurred in tandem with improvements in WMC, but did not appear to be primarily driven by them. Overall, our results indicate that strategy use is a critical underpinning of reasoning performance in children as well as in adults, and that theories of cognitive development of reasoning have to consider the central role of strategy use. (PsycInfo Database Record (c) 2024 APA, all rights reserved).