Tires...Tires...Tires?

I have a set of Michelins on my Bronco that are easily 15 years old. Very minimal cracks in the tread face only, lots of depth still, and I generally only drive to work and back, maybe 10 miles a day. One has gone flat since Friday night so I may pull them off, I have another set in the garage, 2 are nearly new, 2 are getting thin but they should last until spring.