After the escape, a doctor tells Ma that it’s a good thing she got Jack out while he was still "plastic".
What does he mean by plastic?
The scene as described in the script takes place as follows:
DR. MITTAL Have you thought any more about what we discussed this morning?
MA Thanks for the offer but we’re going to go home.
DR. MITTAL You know my view. After what you’ve experienced, and to assess Jack properly. And just to soften the transition -
MA (interrupting) Nothing happened to Jack.
DR. MITTAL I understand, but even at a cognitive-sensory level - depth perception, auditory processing ...
MA But he’s OK, right?
DR. MITTAL The most important thing you did was to get him out while he’s still plastic.
JACK (whispers to Ma) I’m not plastic.
DR. MITTAL What’s that, Jack.
MA He says he’s real, not plastic. Mittal laughs.
DR. MITTAL I can’t argue with that, Jack. Real and very brave.
So it's pretty clear from the context that the doctor is using plastic to imply Jack was young enough that he could cope with leaving "Room" and deal with the world outside (as opposed to an older child or adult who may not be able to cope with this).
To quote one of the definitions of *plastic" from the Merriam-Webster dictionary:
capable of being made into different shapes
That describes the Dr's view quite well.