How so?...besides the loss of grip factor? Tires are round and that is about all I know about them...and how to put them on with my tire machine.
I've heard this about new tires too, especially if they are in the light. Rubber doesn't age well... unlike wine.
I believe what Jim is referring to is how they are stored. If they are left in an unheated garage where temperatures drop below freezing you can get some internal cracking, especially if you not careful when loading or moving them. Ferrari of Washington had a shipment of PZero's a few years ago that arrived in winter time and were badly cracked.
But don't you physically see that as cracks in the sidewalls? And don't you get small leaks if the rubber is violated which is not unsafe just not raceable? And aren't the belts or construction of the tire still fully intact so safety is still o.k.? I would be more fearful of not seeing some damage from either bad PO useage or bad demounting of the tire before I got it. I mount/dismount my own tires so I don't really see houw you can hurt a tire taking it off but maybe you can?