About a month ago I took my truck into the local dealership to have a tire fixed (it had been loosing air). When the mechanic wetted the tire he found little T shaped leakes all over the place between the threads. I know that pattern, it happens with decayed tires.
I had him check the other tires and three of the four leaked. I replaced all 4. The replacements were all made in 08. The leakers were tire that had about 25,000 miles on them and should have been good for at least another 25k. $680 later I had the lesson about old tires driven in thoroughly.
I was lucky, I could have gotten the "old tire lesson" a much more expensive way!
I always watched tread depth and never messed around trying to stretch tread life. Now I will watch the age as well.