[hchasestevens]

That's a little misleading. An individual child will not be able to fluently acquire language if not exposed during the so-called "critical period"; see https://en.wikipedia.org/wiki/Feral_child for plenty of examples.

_Groups_ of humans with no common means of communication will form pidgins ( https://en.wikipedia.org/wiki/Pidgin ) as an (essentially grammarless) means of communication; children then raised in an environment where they are exposed to this pidgin will transform it into a true language. Nicaraguan Sign Language has actually proven to be a fascinating example of this in action; many of the first speakers of the original pidgin are, now that successive generations have learned and adapted it, unable to competently produce utterances in the now-language.

[z3t4]

I we where that stupid we wouldn't be able to learn new languages as an adult.

[vageli]

> I we where that stupid we wouldn't be able to learn new languages as an adult.

That is after successfully having acquired a language already. What the critical period theory suggests is that if no language is acquired during that period, subsequent language acquisition is not possible.