I don't ever recall this topic coming up before, so I'm curious about what others have to say about it too. If your local Pro Shop has a durometer (a device used to check the coverstock hardness), then it would be much easier to check a ball and then go back to the manufacturer's specs to see if the ball has hardened over time. Right off hand, I wouldn't rule out the possibility that a ball might harden over time, but I really can't tell you for sure.