Aging 22.5 tires - how old is too old?

I don't think you can put an age limit on a tire. I have mine inspected every year in March or April, and keep the tire pressure in check. I also do a visual inspection before I hit the road every morning, and just a quick look when I walk around when I stop during the day. Even having done all of that, I can still have a blow out. Mine are 7 years old now, and this last inspection the truck shop told me all was good and I should get a few more years out of them, however, I am changing all 6 out next year to satisfy my comfort level with their age.