The linguistics textbooks will tell you that the unfortunate case of Genie (feral child) demonstrates that puberty delimits a window of opportunity for language acquisition, past which full language acquisition is not possible. Genie learnt enough English for the first time at 13 to communicate, but her English was never grammatical.
There are plenty of horrors surrounding Genie’s case, and quite apart from the monstrous lapses in ethics around how she was handled, any scientific conclusions gleaned from her are problematic.
Genie was cut off from any linguistic or social input, because her father was convinced she was brain damaged. If she wasn’t before, his abuse guaranteed that she was after. If she was handicapped before, which we have no way of knowing, her ability to acquire language may well have been compromised anyway.
On the other hand, she seems to have developed an adequate command of sign language (itself a political football between linguists and psychologists, while they were belly flopping on their professional ethics all around her). Sign language was the psychologists’ idea, not the linguists’, so we don’t really know how much better her sign language was than her spoken language. In any case, once she was released from the lab to endure further physical and sexual abuse, her command of sign language seems to have been impaired as well.
What have we actually learnt about first language acquisition from Genie’s case? Probably not as much as people like to think. And hopefully, we will never have the opportunity to repeat such an experiment again.