How old is this tire?

Most of the manufacturer recommendations run between 6 and 10 years, depending on the manufacturer. I won't run anything older than 10 years based solely on age, but tires newer than that can get dry checking and once that shows up they're junk regardless of how old they are.

So up to ten years I go off condition, after that they're junk regardless of condition, no matter how good they look. But that's for tires I know most of the history on. I wouldn't buy any tires 2nd hand that are older than 5 years.