It is generally accepted, that child who starts learning language at an early age is more likely to learn it perfectly than those who start learning second language after they reach puberty. This idea is called Critical Period Hypothesis. It is connected with the fact that young child’s brain does not lack in some devices which support acquiring first language. It is particularly true for pronunciation which will never be perfect if child starts learning a language after reaching puberty (Lynne 2001:13).
Two versions of this theory can be distinguished. According to the weak version student might still learn language with extreme difficulties and with varying results. On the other hand, the strong version says that if you start learning after achieving puberty student will never become a perfect speaker (Lightbown and Spada 1993:11).