Additional information:
Children acquiring two or more languages simultaneously have been shown to be able to develop grammatical knowledge in each of their languages in essentially the same way as the respective monolinguals: Grammatical systems are differentiated from ear¬ly on; the children proceed through the same phases of development as their monolin¬gual counterparts; they ultimately achieve the same kind of grammatical compe¬tence. If, however, two or more languages are learned successively and the onset of acquisi¬tion falls after the optimal age period for grammatical development, neither the acqui¬sitional process nor its ultimate result share these characteristics with mono- or multi¬lingual first language acquisition. In fact, L2 acquisition by children first expos¬ed to the second language at approximately age 6 or later, resembles adult L2 acquisi¬tion rather than L1 development. These claims about fundamental similarities and differ¬ences between various types of language acquisition are corroborated by linguistic as well as neuropsychological evidence. I interpret such findings as supporting the hypo¬thesis according to which first language development is guided in a substantial fashion by the human language faculty which is, indeed, an endowment for multilin¬gualism but which gradually becomes inaccessible, as a result of neural maturation.