My 89 Honda Accord's compact spare was still quite usable when it had in excess of twenty years of age on it. The car trunk is not exactly a harsh environment.
I raised the air pressure up to 60 PSI like you guys suggested. The tire didn't explode. I remember when I got the tire out of the salvage yard that it still looked brand new and unused. The little whiskers on the tread tell the story. Thank you all you for your replies; very much appreciated.