A lot depends on the environmental conditions and how they affect the compounds used in the various layers of the tire but particularly the outer.
After Pikes Peak was fully paved, Jess Neal won the vintage class using a set of original BFG R1. Key was storage environment. Race compounds are even more senstive to environmental heat cycling than normal compounds so this is an unusual case. However it is a good example that age alone is not the only consideration when assessing condition.