this went around work the other day, and i thought it was common knowledge...it would seem otherwise.......
http://abcnews.go.com/Video/playerIndex?id=4826897
summary if you don;t want to watch the video:
look on your tires and the 4 number code at the end of teh dot markings
signify the date the tires was made you'll see 422 or 2102 or xxx or xxxx
the way the code breaks down is the first 2 numbers are the week so with the above example of 422 it would have been made in the 42nd week the next single or double number designates the year a single number means the 90's a double number means 2000's + so again with the numbers above
the first one with a code of 422 would be the 42nd week of 92
and the second number would be 21st week of 2002
anything over 4-5 years starts to get dodgy, from the article above they were buying tires that where 8-10 years old from discount and other large tires carriers "NEW" of the rack, so when your buying new tires be sure to check the age as it could shorten the life of your "NEW" tires considerably if the rubber has started to break down.
now ask yourself how much you believe this is an issue and make a personal educated guess your self, this is for informational purposes only and not to be used as any sort of gospel, as i have ran tires that are 14 years old for a number of years now with no problems on my vw bus on the other had i've had tires that just sat for 3 years start to crack, use your best judgment personally.
take care all and if this is common knowledge sorry for the post ;-)