Bilinguals are individuals who use more than one language in daily interactions. They do not necessarily have an equally good command of their languages. Age of onset of acquisition (AOA) is probably the most important factor determining whether a native competence is acquired. All L1 children develop full competences in their languages, whereas very few if any L2 learners are able to achieve this goal. This suggests that the LAD guiding L1 is subject to age-dependent changes and is not fully accessible anymore in L2 acquisition. Such changes are reflected in the speech of early successive bilinguals. They use constructions that are also found in the speech of adult L2 learners but not in the language use of monolingual or bilingual L1 children. L1-L2 differences emerge as early as at approximately age 3;6, affecting the morphosyntax of the target language. Maturational changes of the brain are a major though not the only cause of this kind of linguistic behaviour. They open and close windows of opportunities. These constitute senstitive phases during which learners are optimally prepared for certain acquisition tasks. As of approximately age 6–7, children resemble adult L2 learners rather than simultaneous bilinguals.