The effect of chronological age and instruction on music aptitude, as well as the transition between the developmental and stabilized music aptitude stages, were examined to further establish the rationale for selection of the most appropriate music aptitude test for students in Grades 3, 4, and 5. Archived scores of the "Intermediate Measures of Music Audiation" (IMMA) were used in paired t-tests, Wilcoxon Signed Rank tests, and repeated measures ANOVA. No effect of chronological age or instruction was concluded, and a period of transition could not be substantiated definitively. It was conjectured tonal aptitude and rhythm aptitude stabilize independently of one another. However, as type of instruction may have had a deleterious effect on the findings for effect of instruction and substantiation of a transition period, further research is recommended. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]