With something as critical as safety I don't take chances. I replaced the tires I had bought new in 2000 with new ones after 10 years, even though they had plenty of tread left, looked great and had been stored properly (I think, but as Scott says how can I really tell). Did the same with my small trailer last summer. You see so many trailers and caravans stranded by the side of the road with flat tires due to them being used relatively little, so the owner thinks the tires are still ok, when they're not. 10 to 12 years is my max.