IMHO, there is no set age on tires! Every year past 4 years I have my tires broke down and inspected inside and out! I don't care about the cost, what I care about is peace of mind that my tires are good to go after been inspected by a tire expert. To throw out a number of years is a disservice because every tire is different and who knows but you how it's been maintained. If I'm buying a used coach with no tire history I don't care about the miles on the tire what I care about is what the inside and outside of that tire casing looks like from a tire expert. Then, I can make a informed decision based on fact!