Tires: tire age- spare

Question
Hi Barry,
I have a 2005 Ford Escape 4wd with 96,000 miles, bought used. The tires are all Continental Conti Tracs, P235/70R16. The DOT date tells me they were made the 11th week of 2004 (A308 1104) There is plenty of tread on all 4 tires but there is some slight cracking on the side walls, up by the edge of the thread. I also ran my hand around the sidewalls and all 4 tires have a slight depression in approximately the location on each tire?? The Escape was driven in the California desert(dry & hot) for the first 70,000 miles. Having read other related questions on your site, I will probably be replacing the tires, unless desert climates delay aging. But, my real question is the spare tire. It is the same tire, date as those mounted. It is carried under the chassis. It still has lettering printed on the thread surface so it has never touched the road. It shows no sign of cracking, could it be retained as a spare?

Answer
Jerry,

First, desert climates only affect tires because of heat.  A cool dry desert does not cause more issues than a cool wet swamp.  So in terms of age, that tire is toast as far as regular usage is concerned. It's 10 years old!

But as a spare, it could be used for a temporary, short mileage situation.  I'm talking 50 miles here at 50 mph.  And even then it should be rep[laced some years down the road - say 5 to 8.