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I checked another make of urethane tires that I had in storage. There were two pairs of tires still on the aluminum wheels and they had been glued on with superglue. The older pair had turned red, but otherwise had not split or cracked. They crumbled when I poked them however. The other pair was about a year newer and had started to turn red. The older pair had been replaced because they were worn down, we had switched to silicone tires for the class that used the urethanes. I had to check pictures of the car that used those tires to get a date, the tires were 11 years old, they had been stored in a cool dark place. There is no telling when the problem first appeared. I believe that in normal use you would wear out the tires long before the red problem occured.
Right now we are not racing any cars with urethane tires, several years ago we took a swing at an open wheel class that used spec PGT tires, those are now going on four years old. I just checked the three cars with those tires and they are still fine.

This tire has been on this car for several months, maybe 1 year old. It is a PG tire.
The car was NOT raced in weekly races or proxy, just some tire truing and tuning laps every now and then. So it is not like extended time frame or extreme racing conditions like heating and cooling. Lots of humidity here in Hawaii.