My friend’s grandmother never really learned Castilian; she only spoke Catalan. I think she could understand it well enough to watch TV and get the gist of it. We tried to get her to speak it once; she didn’t get very far. I don’t know if she was literate, come to think of it.
But this was thirty of more years ago. Nowadays I suspect you’d be hard pressed to find a native Spaniard who can’t speak Castilian fluently.