You see much the same sort of thing with people who are language-deprived at an early age. Leaving "Genie"[1] out of the mix (because it has been argued by at least one of the people who studied her that she was probably impaired in areas other than language—though I'm inclined to believe that her other cognative impairments were as likely to be the result of language deprivation as the cause of her language impairment), we have seen similar problems with real grammar among the Nicaraguan deaf community: the kids who were older when the schools for the deaf opened never developed a language capacity in the same way that the really young children did. (It has to be kept in mind that Nicaraguan Sign Language[2] is essentially a spontaneous creole developed by the kids themselves from the rough pidgin
mimicas each child brought into the community from home. The grammar of the language came from usage in the school community; it wasn't externally imposed, and isn't related to any of the French-derived sign languages.) Hawaiian Creole English ("Pidgin" to its speakers) can also be demonstrated to be largely the creation of children, building upon the true pidgin (with no consistent grammar) their parents, who came from a variety of quite different languages communities, spoke.
You might think that things like this might reinforce the idea that language capacity is innate in humans, and that it differs in qualitative (and not merely quantitative) ways from communication in other species, but people are still wrapped up in some combination of strict behaviourism and the tabula rasa on the one hand and cultural constructionism on the other hand that they can't seem to see the data sitting in front of them. I don't think we'll get very far until we accept that the communicative aspect of language is merely a lossy format for serializing and transmitting internal data structures that may or may not have any correspondence with physical realities (apart from neuronal state). "A language" is just an agreed-upon format, like JSON, XML, or Gerry Sussman's "everything is a Lisp" notation(s), but without that config file, the general-purpose serializer/deserializer library is just unreachable code.
[1] https://en.wikipedia.org/wiki/Genie_(feral_child)
[2] https://en.wikipedia.org/wiki/Nicaraguan_Sign_Language