Noun-class prefixes are obligatory in most Bantu languages. However, the Sotho languages (Sesotho, Setswana, Sepedi) permit a subset of prefixes to be realized as null at the intersection of "unmarked" phonological, syntactic, and discourse conditions. This raises the question of how and when the licensing of null prefixes is learned. Using longitudinal data from three Sesotho-speaking children, this article shows that the conditions needed to license null prefixes have been learned before the age of three, suggesting early abilities for grammatical generalization even at the intersection of different levels of linguistic structure. The implications for learnability theory and Bantu linguistic structure more generally are discussed.