What makes a semiconductor?
There's no actual hard line I'd be aware of that tells semiconductors from non-conductors in terms of bandgap.

In the end, it's a definition up to the author of any text. It might be helpful to think of things in practical terms (e.g. "can I make a PN junction out of this?"), but that really just brings you further down into subjective statements.

So, 


  Where is the line?


Wherever it's sensible to put it in the context you're working with.