I don't think he had the glass jaw that a lot of people think he did. He had a decent chin. He never got hit much back in the day (and when he did he was usually rolling with it), but I've seen him tagged clean by some decent punchers and take it well.

Modern Jones though, you have to take into account the fact that he's 44 and physically shot at this time, and he's taking bombs from big cruiserweights. And he doesn't have the reflexes anymore to avoid all the big bombs or roll with them.