Instead of batting this tire age thing around yet again with our own opinions I would refer all to the tire manufacturers web sites.I run Michelin RV tires and
This Link takes you to their discussion of tire life for RV tires.
My current tires are 4.5 years old and have been in use for 3.5 years. I see some small sidewall cracks and will have them checked over the next time I have the coach serviced. I will not hesitate to replace between 5 and 6 years because the lives of my wife and grandkids (when traveling with us) depend on the air staying in those tires as we roll at 62 MPH. Just my OPINION. Someone can buy my "lightly worn" used tires and stretch another 4 years out of them, its my money!