Chapter 12. Age of onset in successive acquisition of bilingualism
In first language acquisition, monolingual as well as bilingual, every child develops a full gram­matical competence in the language s/he is exposed to. This is arguably not the case in second language acquisition (L2). My assumption here is that this is due to the fact that the Language Making Capacity which guides L1 development is not fully accessible any more to L2 learn­ers. My claim is that it becomes inaccessible as a consequence of neural maturation, supporting thus the Critical Period Hypothesis. The latter should, however, be understood as a cluster of sensi­t­ive periods, each defined in terms of an optimal period for the development of specific features of grammar. Age of onset of acquisition is consequently argued to be the single most important factor distinguishing acquisition types. As for the age periods at which crucial changes happen, my claim is that they occur significantly earlier than is commonly assumed. More spec­ifically, I will show that linguistic as well as neuropsychological evidence suggests that at least some as­pects of grammar, relating to inflectional morphology and to syntax, are indeed affected as early as at age of onset between age 3 and 4. Further significant changes seem to happen at around age 6 to 7.