Currently most of the industry recommends a 6 year life on tires, mounted or not. The recommendation used to be 10 years, but after the Firestone issues there have been a lot of long boring studies that show the 6 year mark to be relatively "safe." Through NHTSA there is no set tire life recommendation, but I believe the current push in the industry to have NHTSA finally make a set recommendation. I have been to too many accidents lately where people had an older tire or two, older being 8+ years, that either blew out or had a tread separation for one reason or another. In most cases the tire deflation itself wasn't the actual cause of the accident, but the the driver's actions with panic braking or truing to make a quick turn to the shoulder is what lead to the loss of control.
If you don't know how to read the DOT code on the tire, there are several great sources for the info just stay away from anything with only 3 digits in the last series of digits. If you just want to expand your (mostly useless) tire knowledge and have some info for those coffee table conversations, or you care where your tires actually came from, you can look up your tire's plant codes here (http://www.harriger.com/tires.htm
) using the first two digits of the DOT number.