The study explored the extent learners' age variances impacted their achievement in a national education assessment in Ghana and how these were moderated by the types of schools (i.e., private and public) they attended. A multistage sampling method was used, and the data were analyzed using a multilevel modeling technique. The sample comprised 19,210 primary grade 3 and 17,088 primary grade 6 learners from 525 and 499 schools, respectively. Relatively younger learners outperformed their older peers in both subjects except for primary 3 mathematics achievement. Schools marginally reduced the age effect on both subjects except primary 3 mathematics achievement, where there was an increase. Moreover, there was a statistically insignificant difference in private and public schools' impact on age-linked effects on subjects except for primary 3 mathematics. The study concludes that being relatively overage for a specific grade level is not beneficial, especially for English language achievement. Hence, enrolling learners at the prescribed age and school term is highly recommended.