I'm guessing that your tractor has been stored under cover? I don't see any noticeable cracking and the tread looks great on the one in the picture. Those look almost new compared to what I took off mine. Usually what you see with old tires is that the sidewalls start to crack and the treads become soft and tear easily (kind of like punky wood). Even then, many people would continue to run them for years as long as they hold air and have enough tread on them.