Storage environment has a lot to do with the aging of tires. Garage kept, stable temperature, out of the sun, will drastically increase the longevity of the rubber. I've run tires older than 5 years. I've replaced tires younger than 5 years. As always, Your Mileage May Vary.
I replaced my boat trailer tires out of caution as they were 15 years old. Tires looked fine and with plenty of tread, but I could feel the rubber was harder than newer tires. I didn't want a tread separation to ruin a good Saturday of fishing.