Is there an ideal time window for language acquisition after which nativelike representation and processing are unattainable? Although this question has been heavily debated, no consensus has been reached. Here, we present evidence for a sensitive period in language development and show that it is specific to grammar. We conducted a masked priming task with a group of Turkish-German bilinguals and examined age of acquisition (AoA) effects on the processing of complex words. We compared a subtle but meaningful linguistic contrast, that between grammatical inflection and lexical-based derivation. The results showed a highly selective AoA effect on inflectional (but not derivational) priming. In addition, the effect displayed a discontinuity indicative of a sensitive period: Priming from inflected forms was nativelike when acquisition started before the age of 5 but declined with increasing AoA. We conclude that the acquisition of morphological rules expressing morphosyntactic properties is constrained by maturational factors.