The best evidence that there is a critical period for language acquisition is the fact that

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The fact that people most easily master the grammar of a second language during childhood is the best evidence that there is a critical period for language acquisition. Studies have shown that as children get older, it becomes more difficult for them to learn a second language and to master its grammar. This suggests that there is a critical period in childhood during which language acquisition is easiest and most efficient, after which it becomes more challenging.

The Best Evidence That There Is a Critical Period for Language