Proper storage does squeeze a little more life out of them but the rubber itself still oxides and dries out leading to reduced performance.
You can read the year/month date code on the sidewall. I myself wont put anything on that is more than a year or 2 old at most from new. This is assuming they will be on the car for a few years.
Actually auto-xers are probably the most picky about the date codes. Most of the hard-core folks won't even buy the discount tires in the winter, they wait for the fresh spring stock to come out!