Plastic?

Plastic implying fake, artificial?

I don't know.

The doctor saying 'plastic' refers to the malleable development of the brain during youth, there's a critical window of time in which humans learn certain advanced concepts like language, if they miss this window they will never be able to learn these things or won't be able to learn them to as high a degree.

Trivia Section:
"While they are still in the hospital, a doctor tells Joy that she is lucky to have gotten Jack out of captivity while he is still "plastic." In psychology and child development, "plasticity" refers to a child's ability to learn, develop, and recover from trauma. Neuroplasticity, specifically, would be vitally important in ensuring Jack is able to successfully interact with the larger world. Humans of all ages have varying degrees of neuroplasticity."

"Feral Children...
lack the basic social skills that are normally learned in the process of enculturation. For example, they may be unable to learn to use a toilet, have trouble learning to walk upright after walking on fours all their lives, or display a complete lack of interest in the human activity around them. They often seem mentally impaired and have almost insurmountable trouble learning a human language.[1] The impaired ability to learn a natural language after having been isolated for so many years is often attributed to the existence of a critical period for language learning, and taken as evidence in favor of the critical period hypothesis."
https://www.wikiwand.com/en/Feral_child