After experiencing a flat tire on my other trailer recently, the tire dealer I bought the new tires from, told me that trailer tires usually only last about 5 years. They tend to dry out, and the rubber starts to go hard, causing blowouts when they heat up while towing at hwy speeds.
I figured I better check the tire age on my 94 Heritage trailer, since we plan to tow the boat around a fair bit this summer, and I dont want to be stuck on the hwy trying to fix a flat again...
Well low and behold, I found out that my trailer has the original tires on it, that were made back in 93.

Despite the fact that there was plenty of tread remaining, there were numerous small cracks all the way aroundin the treads.
Needless to say I replaced all 4 tires, and my wallet is about $500 lighter now, but I do have the piece of mind knowing I have all new rubber. Baring any punctures from nails or what have you, I should be good for another 5 years.