The door hinges inside my place are pretty old; I'd guess a few decades though I don't have any way of knowing for sure. I do have experience with metal on metal rubbing and degradation. So I wonder, why can door hinges open and close for a long time, and not wear away? I don't think I've ever seen a door hinge that was worn. It seems like just a little bit of wear would result in the door not opening as smoothly as it should.