In this paper we argue that investigation of third language (L3) acquisition by adults and children provides essential new insights about the language learning process that neither the study of first language (L1) nor second language (L2) acquisition alone can provide. The focus of this paper concerns the role the learner's L1 plays in succeeding language acquisition. Specifically, does the L1 maintain a privileged role in all subsequent language acquisition or is it possible that all languages known can play a role in subsequent language acquisition? Results from several ongoing L3 acquisition studies investigating adults and children learning English as an L3, whose L1 is Kazakh and whose L2 is Russian, are reported in this paper. We compare the learners' patterns of acquisition in production of three types of restrictive relative clauses. Adults are compared to children; speakers are compared at comparable levels of linguistic competence in the languages. Results indicate that the L1 does not play a privileged role in subsequent acquisition. At the same time, results indicate subtle differences between adults and children. We argue that such findings would not be possible in the context of an L1 or L2 study alone. Results are used to support the proposed "Cumulative Enhancement Model for Language Acquisition." (Contains 6 tables, 1 figure and 2 notes.)