In the U.S., The Dept of Transportation requires a "date of manufacture" code on all tires. It is on one sidewall, in a little flat area about 1/2 inch by 1 inch. The code has two 2 digit numbers, like 51 06. What this means is the tire came off the manufacturing "line" the 51st week of 2006.
At least for U.S. bound tires, every maker uses the same coding system. Sometimes you may see the letters DOT followed by the numbers.
My tires on my 01/07 made bike are 05 06, meaning they were almost a year old when I bought my bike on 03/16/2007, and they are now well over a year old.