The purpose of this study was to examine developmental change associated with mature striking by children in grades K-8. Specifically, we addressed the following research questions: What are the characteristics of individual differences in mature striking development over time? Are differences in developmental growth trajectories of mature striking systematically related to age, sex, or participation in organized sports? The basic design for this study was multicohort longitudinal. This design enabled us to examine the association of age, sex, and baseball/softball participation while reducing cohort effects. We tested children's performance on the striking subtest of the Ohio State University Scale of IntraGross Motor Assessment (OSU-SIGMA). More precisely, we tested four cohorts of boys and girls in grades K-8 (N=340) from 1992 to 2000: 1992=K-8, 1994=2-8, 1996=4-8, and 1998= 6-8. In 2000 we again tested the original kindergarten cohort. The data were analyzed by hierarchical nonlinear modeling. This technique uses slopes and intercepts of children's striking performances to establish individual growth trajectories. Predictor variables (age, sex, and baseball/softball participation) were then added to the model to provide a clearer understanding of mature striking development. The principal findings were: (a) significant differences were observed in mean initial status (intercept) for the 2-8 and 4-8 cohorts; (b)significant differences were observed in growth rate (slope) for the K-8 and 2-8 cohorts; (c) sex differences were observed in initial status and growth rate in the 2-8 cohort, and (d) no differences were observed for participation in baseball/softball.Keyword(s): elementary education, gender issues, middle school issues