I read somewhere that bones harden as they dry out and a dog is more likely to break a tooth on an old hardened bone.
Dogs can eat some disgusting stuff, so I don't think they'll get sick.
I make a habit of picking up bones every day so I don't forget about them that way I don't have old dried out bones laying around, no doubt I've missed a few here and there, I reckon he has a secret stash or several stashed in shallow holes around the place.
I had trouble finding a butcher that would sell me the actual brisket which is really cartilage (there is some confusion over the true definition of what a brisket actually is), they all sold me a bit of brisket and most of it was sawn up rib bones which my dog couldn't eat.
This link has some info on what a brisket is (to me anyway)
my dog: cuts of meat for dogs to eat