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The critical period hypothesis is the subject of a long-standing debate in linguistics and language acquisition over the extent to which the ability to acquire language is biologically linked to age. The critical period hypothesis states that the first few years of life is the crucial time in which an individual can acquire a first language if presented with adequate stimuli. If language input does not occur until after this time, the individual will never achieve a full command of language—especially grammatical systems. The evidence for such a period is limited, and support stems largely from theoretical arguments and analogies to other critical periods in biology such as visual development, but nonetheless is widely accepted.
In second-language acquisition, the strongest empirical evidence for the critical period hypothesis is in the study of accent, where most older learners do not reach a native-like level. Lenneberg’s critical period hypothesis states that there are maturational constraints on the time a first language can be acquired. If language acquisition does not occur by puberty, some aspects of language can be learned but full mastery cannot be achieved. Support for the critical period theory stems largely from theoretical arguments and analogies to other critical periods in biology such as visual development. Recently, it has been suggested that if a critical period does exist, it may be due at least partially to the delayed development of the prefrontal cortex in human children. Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages. While the window for learning a second language never completely closes, certain linguistic aspects appear to be more affected by the age of the learner than others.
For example, adult second-language learners nearly always retain an immediately identifiable foreign accent, including some who display perfect grammar. The critical period hypothesis in SLA follows a “use it then lose it” approach, which dictates that as a person ages, excess neural circuitry used during L1 learning is essentially broken down. If these neural structures remained intact they would cost unnecessary metabolic energy to maintain. The structures necessary for L1 use are kept.