I just bought a 250' roll of 12/3NM for $15. The trick is that it is really
old. The cable bends back and forth nicely without anything happening to
the insulation, but the box is crumbling.

Is this okay to use, or might age have done something to it. Yeah, I know
installed 40 year old cable is just fine, but this will be bent up some
during installation, so it might not be the same thing.
Anyone know?

They had a second roll, but I don't think I will ever go through this one.
I just sold 420' of 14/2 (for $85) because I had a major wire surplus from
garage sales; I just can't resist a deal.