It might depend more on how they are stored in the off season than how many physical miles are on them, unless they were loaded heavy and under inflated.
A "D" rated tire likely has steel belts in it somewhere. From the various posts over the years, is seems that the rubber in tires absorb moisture from the ground and the metal wire corrodes creating a bad bond to the next layer (I'm suspicious of the nylon layers as well, but picture a rusty wire in the middle of the tire). Somewhere down the road, they will start to create a bubble since their bond is now broken, and delaminate. Ozone causes cracks and this will also hasten the tire break down as well. If the tires are covered in the summer, kept up off the dirt and kept properly inflated, they should last a while. The tire industry is using a 6 year max life to ensure that you are running the safest tires you can. Ask your tire guy to show you how to find the manufacturer date on the tires (or google it) so you know at least how old the tires are to start with.
If you have to ask, you are starting to question them, so you might do yourself a favor and start shopping, or at least make sure you have plenty of spares.
Went to Shasta one night with a guy and he had 2 spares on board. We lucked out, we had 3 flats in 20 miles that night, and fortunately Wal-Mart was still open to fit us for the 3rd flat. We stopped by Les Schwab in the morning and got a set of 4 new (plus free BEEF being February).
