Tires have a manufacturing date code molded into the side wall - on one side. Yours probably says something like "4206". That would mean it was made is the 42nd week of 2006 - it's now 8 years old. If you look carefully it likely has some small cracking in the roots of the tread. If so, as crimedog mentioned the rubber has dried out, it's hardened, and it's not safe.