My former coworker just posted a great quote that her daughter said:

“Shark boy and Lava girl are incompatible.”

Clearly, she was referring to this

“Incompatible” is a pretty big word for a five year old. She has access to this sort of language so she can try it out. I wonder how much of her saying this was testing the word to explore what it means. (It’s in her Zone of Proximal Development)*

Compare this sentence to the average utterance in an AAC system. When was the last time you programmed something this interesting and complex into an AAC system? (I know I never have.)
The point of all of this is that kids won’t learn this sort of language unless they have access to it. Don’t wait until they are “ready” and then give it to them. Throw the kids into the deep end of the linguistic pool and let them construct their way out.

*(This little girl’s mother is a behavior analyst and probably doesn’t hold to the theory of ZPD or language construction, but that’s beside the point).