The first language speakers of any language expose to their mother tongue in the early stages of life. In the long term and in immersion contexts, second language learners also start the acquisition process in early stages of life; and stay exposed to input and thus and learn over several years or decades; undisputedly tend to outperform later learners. Apart from being misinterpreted as an argument in favor of early second or foreign language instruction, which takes place in wholly different circumstances, this general age effect is also sometimes taken as evidence for a so-called “critical period” (CP) for second language acquisition (SLA).
Derived from biology, the CP concept was famously introduced into the field of language acquisition by Penfield and Roberts in 1959 and was refined by Linneberg eight years later. Linneberg argued that language acquisition needed to take place between age two and puberty; a period which he believed to coincide with the lateralization process of the brain. More recent neurological research suggests that different time frames exist for the lateralization process of different language functions; mostly, however, close before puberty. However, Linnerberg mostly drew on findings pertaining to first language development in deaf children, feral children or children with serious cognitive impairments in order to back up his claims. For him the CP concept was concerned with the implicit “automatic acquisition” in immersion contexts and does not preclude the possibility of learning a foreign language after puberty. However, Johnson and Newport (1989) offered two different interpretations for the hypothesis regarding age affect role in L2; Exercise hypothesis and the Maturational hypothesis.
The two different interpretations for CPH presents two different implications for the role of age effect in L2 acquisition. According to the prediction of the Exercised Hypothesis, children will be superior to adults in acquiring a first language; if learners are not exposed to a first language during childhood, they will not be able to acquire L2 fully at a later time. However, as long as they have acquired a first language during childhood, the ability to acquire L2 will remain intact and can be utilized at any age. On such a hypothesis, L2 acquisition should be equivalent in children and adults, hence there will be no age effect role in L2 acquisition. In contradiction, the Maturational State Hypothesis claims that there is something special about the maturational state of the child’s brain which makes children particularly adept at acquiring any language, first as well as L2. With time going on, the special maturational state will not keep intact and the abilities to acquire L2 will decline with maturation, regardless of whether exercised or not. On such a hypothesis, age effects can be observed in L2 acquisition.
SLA related research adopted the critical period hypothesis and applied it to second and foreign language learning, resulting in a host of studies. In its more general version, the “Critical Period Hypothesis” for second language acquisition states that the ‘susceptibility’ or ‘sensitivity’ to language input varies as a function of age, with adult L2 learners being less susceptible to input than child L2 learners. Importantly, the age susceptibility function is hypothesized to be non-linear. Moving beyond this general version, it is sated that the CPH is conceptualized in a multitude of ways. This state of affairs requires scholars to make explicit their theoretical stance and assumptions, but has the obvious downside that critical findings risk being mitigated as posing a problem to only one aspect of one particular conceptualization of the CPH, whereas other conceptualizations remain unscathed. This overall vagueness concerns two areas in particular; the delineation of the CPH’s scope and the formulation of testable predictions. Delineating the scope and formulating falsifiable predictions are, needless to say, fundamental stages in the scientific evaluation of any hypothesis or theory, but the lack of scholarly consensus on these points seems to be particularly pronounced in the case of the CPH.
