The development of divergent thinking (DT) in school-age children and adolescents has received considerable attention in the educational psychology literature since the 1970s. A body of research has outlined the existence of slumps (i.e., temporary declines) in this development with, however, conflicting findings concerning the magnitude and timing of these slumps. This study is the first to meta-analyze prior research findings regarding DT development from Grades 1 to 12, with a particular emphasis on the widely controversial fourth-grade slump. A total of 2139 standardized means from 41 studies involving 40,918 subjects were analyzed using a meta-analytic three-level model. The findings showed an overall upward developmental trend of DT across grade levels, with some discontinuities. Specifically, there was no evidence of a general fourth-grade slump; rather, evidences for a seventh-grade slump were found. Moderator analyses indicated that a fourth-grade slump might be observed depending on DT test, task content domain, intellectual giftedness, and country of study. The existence of the seventh-grade slump was also moderated by DT test, task content domain, and gender. Together, this study deciphers a longstanding debate regarding DT development, a prerequisite knowledge to support age-appropriate educational strategies that encourage creativity development. Implications of these findings for creativity research and practice are discussed.