I just ordered a set of Carlisles for my rig.
The tires that were on it were the original ones from the 36th week of 2012.
If anyone has any doubt that tires will only last 5 yrs, this was proof enough for me.
The tread was in fine shape and hardly worn, but on close inspection, the tires themselves were cracking quite noticeably, or I guess "weather checked" as they say.
Glad I got to them before they got to me