Seems a funny answer from them. First of all it doesn't take that long shipping tires from Taiwan. They just had them that long in stock what's seems unusual, unless the warehouse people used the fresh stock coming in.
Tires even degrade some in the warehouse because rubber get harder over the years. But since they where not subject to UV-rays they still will go for your 5 years of service and even longer. But since the Tire manufactures want your money they recommend 5 years. Yes here in Canada is a supplier who recommends 3 years for Trailer Tires.
2014 Ram 1500 CrewCab 4x4 5.7 Hemi 3.92 Rear and Air Lift 1000
2005 Jayco Jay Feather LGT - 29Y GVWR-6500 lbs.
Pro Series 1200 lbs. WDH with Double Sway Bar.
Champion 3100/2800 watt Inverter/Generator-Onboard Solar Power
ALJO 'Forever Young @ Heart' provide on their travels Music Entertainment to Senior Care Homes