The critical period hypothesis (from Wikipedia):

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 hypothesis claims that there is an ideal time window to acquire language in a linguistically rich environment, after which further language acquisition becomes much more difficult and effortful.

In second-language acquisition, the strongest evidence for the critical period hypothesis is in the study of accent, where most older learners do not reach a native-like level.

Is it true that there is an ideal time window to acquire language, after which further language acquisition becomes more difficult?

Do most older learners never acquire a native-like accent in a second language? Do most older learners also never acquire native-like grammar?

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    I do recognize that a few people have been able to achieve native-like fluency to the point that natives mistake them for one. But I just want to know if this level of fluency is accessible to the majority of language learners, and not just geniuses. Oct 7 '13 at 6:29
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    Anecdotally, yes, for example if you marry someone and live in their country for a couple of decades.
    – ChrisW
    Oct 7 '13 at 10:46
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    I strongly believe that it is totally possible for 90% of people and I've seen several people to do that. But I cannot prove it.
    – user11212
    Oct 7 '13 at 12:25
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    See Bilingualism in Development: Language, Literacy, & Cognition by Ellen Bialystok, pages 71-87, for a modern account that's critical of the critical period hypothesis.
    – user8949
    Oct 7 '13 at 12:25
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Before I get into detail, I'd like to make a few remarks regarding the questions and their possible pitfalls.

First of all, it is not easy to assess how difficult it is for young children to learn a language because they cannot provide you with cognitive feedback until a certain age. That is why "more difficult" lacks a starting point that it can be compared to and in any case there is still a lot of ground for subjectivity when it comes to difficulty.

As to the question about whether most older learners never acquire a native-like accent, the answer is most likely 'yes'. But I strongly believe it is not the result of some critical period but rather the external factors and approach. For more information, see below.

The CPH (Critical Period Hypothesis) popularized by Eric Lenneberg has been the subject of lively debate among scholars ever since its conception. In the case of first language acquisition,

<...> the hypothesis is difficult to test directly because cases of linguistic deprivation during childhood are fortunately rare. <...>

Many researchers have hypothesized that young children are predisposed to the acquisition of language (Chomsky, 1959; Lenneberg, 1967; Newport, 1990) and further that this disposition is unique to childhood. Lenneberg (1967) formally proposed a critical period for language acquisition that extends from infancy until puberty. Although the exact timing (Johnson & Newport, 1989; Krashen, 1973; Lenneberg, 1967) and nature (Lenneberg, 1967; Newport, 1990) of the critical period are matters of debate, there is a great deal of indirect evidence to support the hypothesis (Basser, 1962; He´caen, 1976; Johnson & Newport, 1989, 1991; Lenneberg, 1967). Direct evidence, in the form of individuals who were deprived of linguistic input during the proposed critical period, is more rare, and the interpretation of these cases is often ambiguous (Curtiss, 1977; Mayberry & Eichen, 1991; Skuse, 1993). (1)


...it should be noted that, while the postpubescent learners did not reach as high a level of proficiency as the native or early learners, language had not become totally unlearnable for them. This rules out any extreme interpretation of the critical period hypothesis. (2)

In the case of second language acquisition, the evidence is mixed.

Scovel's (2000) review indicates that during the 1980s expert opinion swung away from the CPH. It was influenced no doubt by a number of negative evaluations of early language learning at school (e.g. Burstall et al, 1974) which indicated that the initial gains from making an early start at primary school had largely evaporated within a few years at secondary school. In recent years however opinion has begun to swing back again. Initially the CPH focused mainly on speech (native-like accent) but in recent years has been extended to embrace other aspects of language competence such as grammar (particularly morphology and syntax), opening up the possibility that there may not be one 'critical period' which applies at the one time 'across the board' but that different aspects of language competence may go through different periods which are particularly sensitive for their development.

Marinova-Todd et al 's (2000) review, on the other hand, is more sceptical about the CPH, pointing to thirty-five fairly recent studies, of which fourteen seemed to offer some support for the CPH, with twenty-one providing negative evidence. Several of these studies, they claim, show that learners post-puberty are in fact capable of achieving native-like competence. In particular they claim that those favouring the CPH have committed three fallacies: misinterpretation, misattribution and misemphasis¹.(3)

The excerpt has an interesting footnote that elaborates on possible misconceptions arising from what seems obvious versus what may be the case:

¹ Marinova-Todd, Marshall and Snow (2000) accept that generally adults achieve lower levels of proficiency than younger learners do, but they attribute this to contextual rather than to biological factors. They claim that those favouring the CPH fall victim to three fallacies: 'The first fallacy is misinterpretation of observations of child and adult learners, which might suggest that children are fast and efficient at picking up second languages. Hard data make it clear that children learn new languages slowly and effortfully - in fact, with less speed and more effort than adolescents or adults. The second fallacy is misattribution of conclusions about language proficiency to facts about the brain; connections between brain functioning and language behaviour will no doubt in time be confirmed, but their exact nature cannot even be guessed from the data currently available on brain functions in early versus late bilinguals. Finally, the common fallacy of reasoning from frequent failure to the impossibility of success has dogged second language research. Most adult second language learners do, in fact, end up with lower-than-native-like levels of proficiency. But most adult learners fail to engage in the task with sufficient motivation, commitment of time or energy, and support from the environments in which they find themselves to expect thigh levels of success. …… this misemphasis has distracted researchers from focusing on the truly informative cases: successful adults who invest sufficient time and attention in second language acquisition and who benefit from high motivation and from supportive, informative second language environments.(3)

I will address the question about native-like accent with the following citation:

A number of recent studies, e.g. Bongerts et al (1997), Nikolov (2000a), Bellingham (2000), and Neufeld (2001) all suggest that adults are in fact capable of attaining a native-like accent, which runs counter to the CPH. Nikolov's study featured thirty three successful language learners aged 20 to 70, all of whom had acquired their target language after puberty. Of these, twenty were of different first languages learning Hungarian and thirteen were of Hungarian as first language learning English. She found that, as judged by three groups of native speakers, six of the learners of Hungarian and five of the learners of English were either generally or often mistaken for native speakers, and she concludes that this calls any strong version of the CPH into question. Her survey showed that 'these successful language learners want to sound like natives, they share intrinsic motivation in the target language which is often part of their profession, or they are integratively motivated.(3)

Another paper that you might be interested in reports on two studies that carefully screened a group of successful late second language learners that mastered a native-like accent: SSLA, 19, 447-465. Age and Ultimate Attainment in the pronunciation of a foreign language. By Theo Bongaerts, Chantal van Summeren, Brigitte Planken, and Erik Schils

(1) BRAIN AND LANGUAGE 63, 237–255 (1998) ARTICLE NO. BL971943 First-Language Acquisition in Adolescence: Evidence for a Critical Period for Verbal Language Development. By Gina M. Grimshaw, Ana Adelstein, M. Philip Bryden,† and G. E. MacKinnon.

(2) COGNITIVE PSYCHOLOGY 21, 60-99 (1989) Critical Period Effects in Second Language Learning: The Influence of Maturational State on the Acquisition of English as a Second Language. By JACQUELINE S. JOHNSON AND ELISSA L. NEWPORT

Guide for the development of Language Education Policies in Europe From Linguistic Diversity to Plurilingual Education. Reference Study. By Richard JOHNSTONE. 2002

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