The Critical Period Hypothesis
The facts of adult second language acquisition (L2A) contrast sharply with those of first language acquisition (L1A). Whereas the attainment of full linguistic competence is the birthright of all normal children, adults vary widely in their ultimate level of attainment, and linguistic competence comparable to that of native speakers is seldom attested. A reasonable explanation for the facts of L1A and L2A is given by the Critical Period Hypothesis (CPH).
In its most succinct and theory-neutral formulation, the CPH states that there is a limited developmental period during which it is possible to acquire a language, be it L1 or L2, to normal, nativelike levels. Once this window of opportunity is passed, however, the ability to learn language declines.
Consistent with the CPH are the morphological and syntactic deficits of Genie, who was largely deprived of linguistic input and interaction until age 13 (Curtiss, 1977), as well as the desultory linguistic achievements of most adult L2 learners.
The Fundamental Difference Hypothesis
The difference in process and product of child, as opposed to adult, acquisition, is a topic that has been widely discussed and continuously debated, often in terms of the Fundamental Difference Hypothesis (Bley-Vroman 1989, 1990). This proposal claims that child L1A and adult L2A are essentially distinct in terms of initial state, process and outcome, and it suggests that the reason for this is the availability of UG for the former but not for the latter.
The adult/child difference is closely linked to a second hypothesis, that of the Critical Period (lenneberg 1967) proposing that first language acquisition is biologically determined to be inevitable, inexorable and chronologically delimited to a sensitive period (multiple critical ages) during the childhood years.
The proposal has spawned ample investigation and controversy between those claiming adult L2A to be abject failure and those pointing to adults who gain native-like proficiency.