There is no guessing involved; you DO know the age of those tires. It is stamped on the side as a 4 digit number code per Department of Transportation (DOT) regulations. The number, lets say 1712 indicates that the tire was born on the 17th week (out of 52 total) of 2012. You can read about it at the link below. Frankly, I have never seen the DOT code written the way Tire Rack has it portrayed in their link (with LMLR preceding the number in the oval), but I thought I'd include their example in case your tires resemble their example. Also, you may have to crawl underneath the car in case the tire was mounted in such a fashion that the code is on the side wall facing in. I believe the code is supposed to face outward to make it easy to read however. Always remember, age kills cars. In the case of tires, the sun does the damage. If the tires are older than 8 years, they really should be replaced. The depth of tread does not matter on an older tire that could suffer a fatigue failure (inability to disperse heat, or hardened/cracked rubber) at any moment:
From what I've read, it's not _sun_ that kills the tires. It's heat - and exposure to air.
The various sites (including manufacturers) all say that for off season tires, to keep them good longer, wash them, then bag them in plastic, dry. Sealing them in keeps the volatiles necessary for flexibility intact. So you can keep your tires longer than normal.
Tires in use generally last less than 4 years - then they'll show signs of cracking. My spare (an 07) is still in good shape, but it's been reasonably sealed up. The one from my 03 disintegrated. Neither one was exposed to the sun.
So, if someone offers used tires, they'd better be VERY cheap.