OK; Here's one for all of you technical gearheads.

I recently read an article in one of the bike rags about how you should replace the tires on sportbikes every 4 years or so, even if the there's plenty of tread depth remaining. Evidently the rubber hardens up with age and causes a serious decrease in traction, to the point where it may be actually dangerous to corner hard.

This sounds like good sensible advice, but it got me to thinking...

When I'm plunking down good money to purchase new tires (especially through the less-expensive mail order houses), how do I know that the rubber is any "fresher" than the crusty old donuts on my bike? I can easily envision the "new" tires having sat in some warehouse for who knows how many years, and already being hardened before I even get a chance to mount them.

Then again, what exactly causes tires to harden? I'm sure UV exposure comes into the picture, as well as repeated heating/cooling cycles. Both of these probably wouldn't happen in a warehouse, unless the tires were stored in direct sunlight, in a non temperature-controlled area.

I've also heard that ozone is particularly nasty to rubber, and is generated indoors by electric motors (refrigerators, compressors, etc).

So, my question is this. How likely is it that tires stored for several years will harden to the point where their traction is decreased enough to matter? Are there any storage precautions that should be taken to prevent this hardening? What is the preferred method of tire storage, and how long can one expect the rubber to remain fresh?