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). The 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. The 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, the claim is that they occur significantly earlier than is commonly assumed. More specifically, it is shown 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.