Age of tires on your tow vehicle/ When to replace?

A timely thread. Went to bring the rig home from storage last week and a tire was flat on the KarKaddy. The Goodyear Marathon flat tire had 1,400 miles on it since it was the spare I installed last May to replace some no name weird brand we got in a pinch when another Goodyear Marathon tread separated on I40 somewhere east of Santa Rosa five years ago. I hit it with a compressor and heard a loud hissing sound. Great, probably picked up a leak from a nail. Pulled the tire and found a 4 inch long by 2 inch wide clean tread separation hanging on in the middle of the tread of this new tire. Talk about lucky since I had not used the Kaddy since parking it after the trip in May. The local dealer who handles Goodyear looked at the tire and told me it was 6 years old and Goodyear only gives 3 years on trailer tires. Kiss off. Lesson learned and no more Goodyear trailer tires.