Nylon tires withstand age much better than radials. I have seen a lot of 40 year old fabric tires still roadworthy. Radial tires begin coming apart with age. I have about 100 radial tires on storage trailers and some just in a warehouse where they were forgotten about. They start by humping up in the center tread and then suddenly blow out. I try not to run tires over 4 years because blowouts cause serious damage to aluminum trailers. Date codes are clearly stamped on tires. As for keeping them covered up, I had a spare tire in a Mercedes wagon blow. It was 12 years old. I put a size larger on the ground and as far as I know, the spare was never exposed to daylight in its entire life. It sure made itself some daylight in the back of the wagon. I'm glad it didn't happen while my wife was driving. I agree with Scott. Drive them for 10 years and get some use out of them. Then get rid of them