I have 2 sets of tires for my Miata.  Last Tuesday I drove my Miata all day on my winter tires and was surprised how slippery they were.  So, I got gas at my favorite tire store, Middle Valley Tire and looked at the build dates.  All of them were 2010!!!  No wonder they were hard and slippery!  Fortunately, they are almost down to the wear bars.  New ones ordered.  Some folks say you should replace tires at 6 years.  Don't know but 10 is plenty long enough. 

The date code is the last 4 digits after "DOT".  The first 2 are the week and the last 2 are the year.