According to some researchers, the defining difference between a first language (L1) and a second language (L2) is the age the person learned the language. For example, linguist Eric Lenneberg used second language to mean a language consciously acquired or used by its speaker after puberty. In most cases, people never achieve the same level of fluency and comprehension in their second languages as in their first language. These views are closely associated with the Critical Period Hypothesis.
In acquiring an L2, Hyltenstam (1992) found that around the age of six or seven seemed to be a cut-off point for bilinguals to achieve native-like proficiency. After that age, L2 learners could get near-native-like-ness but their language would, while consisting of few actual errors, have enough errors to set them apart from the L1 group. The inability of some subjects to achieve native-like proficiency must be seen in relation to the age of onset (AO). The age of 6 or 8 does seem to be an important period in distinguishing between near-native and native-like ultimate attainment¦ More specifically, it may be suggested that AO interacts with frequency and intensity of language use (Hyltenstam, 1992, p. 364).
Later, Hyltenstam & Abrahamsson (2003) modified their age cut-offs to argue that after childhood, in general, it becomes more and more difficult to acquire native-like-ness, but that there is no cut-off point in particular. Furthermore, they discuss a number of cases where a native-like L2 was acquired during adulthood.
As we are learning more and more about the brain, there is a hypothesis that when a child is going through puberty, that is the time that accents start. Before a child goes through puberty, the chemical processes in the brain are more geared towards language and social communication. Whereas after puberty, the ability for learning a language without an accent has been rerouted to function in another area of the brain”most likely in the frontal lobe area promoting cognitive functions, or in the neural system of hormone allocated for reproduction and sexual organ growth.