For the reasons MNBoxster states, the general concensus among tire manufacturers is 10 years for any tire and 6 years for unmounted tires. But it should be noted that this can vary from tire to tire.
http://www.ford.com/en/innovation/safety/resources/tireSafety.htm
Look under "age", Ford has good reason to be cautious about this too.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=138