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Children most easily adopt languages before the 12th year of age. Between the ages of 12 and 14 language acquisition becomes more difficult, with the majority failing to obtain native(accentless)fluency after the age of 16. http://en.wikipedia.org/wiki/Critical_period_hypothesis

Children and adults have been shown to retain this ability for the acquisition of a third language until the age of 50, if they have obtained accentless fluency in a second language before the age of 16.