Aging 22.5 tires - how old is too old?

Yep, varying ops and for me, bought new that had GY G670's. Took delivery Dec '06 and have maintained tires all around. Last year, had sidewall belt break and started coming thru the side(up front). And with the rivering issues, I wanted and put two XZA2's up front...........what a noticable difference. Now, a couple of weeks ago even after checking press/treads/etc, had a blowout on NJ Turnpike on inside dual. Looking at the tire once off, it looked like sidewall blowout and then tread just stripped off. Got two new bridgestones(only ones could find on a sat(wanted XZA's). This week ordered the other 4 XZA's and will put across the drive axle and move the bridge's to tag and spin balance all rears. Then heading to Alliance Coach(old Monaco)in lakeland/wildwood to repair fiberglass. At least blowout didn't take out the radiator. For the record, there is still plenty of tread on the G670's and hopefully will be able to see these Michelins in 6-7 years.