According to Wilder Pendfield, Critical Period Hypothesis (CPH) defines that it is ONLY Possible for a person to learn either L1 or L2 fluently before the age of 9.
But I have met people, who have developed English in both ESL and EFL context with equal native-like fluency in all skills after their twenties.
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A). Have you come across anyone have developed his/her English considerably fluently after Teens or twenties?
B) How much do you agree to Critical Period Hypothesis, and briefly explain why?