The present study examined the relative salience of age within cohort, grade retention, and delayed school entry (3 dimensions of age appropriateness) in 3,684 high school students' academic motivation, engagement, and performance. Structural equation modeling revealed that after the effects of demographic characteristics and grade retention were taken into account, little significant variance was explained by the linear effects of age within cohort. However, subsequent modeling incorporating nonlinear effects showed that the markedly older-for-cohort students (i.e., over the "standard" 12-month age range for a given cohort) and delayed-entry students (i.e., academic "red shirts") experienced some academic disadvantage in motivation, engagement, and performance while the age-appropriate students (particularly the younger ones) fared best. Over and above demographic and age-within-cohort effects, the effects of grade retention were consistently negative. Taken together, data suggest that there appear to be little or no motivation, engagement, or performance advantages to being markedly older-for-cohort, having delayed-entry status, or being retained in a grade. (Contains 2 figures and 2 tables.)