Actually, they're pretty well past their "sell by" date. 15" Dirt Stockers haven't been made in, what, about 8 years? I bought "new" non-DS rain tires in 2006, so somewhere around then...IMO, even if those things were kept in a vacuum, sunlight-free, hermetically-sealed room, they won't be as good as new HO2s...they're better than treaded street tires, but not too much so.
Look for an insert in the sidewall of the tire, one side. You'll see a four digit number that specifies the week and year of manufacture. So something that say 3406 means it was made the 34th week of 2006.
GA