When I got my fintail I noticed some strange cracks and bulges in the tires after a few months of driving. Turns out they were 21 years old! I ordered new tires immediately. The new ones are much nicer, although I still have the 40+ year old bias ply spare in the trunk.

If you plan to drive your vehicle frequently, or above 45 miles an hour, I'd recommend replacing your tires. Otherwise, inspect them carefully before each drive.