There's so much more to consider than just age. Quality of the tire manufacturer to begin with is huge, then how they have been used, mileage, weight while sitting, sitting in direct sunlight, temperature extremes, etc.
I've got BFG radials on my hot rod Model A that were installed December of 1997 (17 years old) that have less than 5,000 miles on them, have been inside my garage out of direct sunlight except when I'm driving it and they have had tire dressing put on them numerous times. I drove it just the other day and they are fine...
