I agree with "replace after 6 years regardless of wear". Tires actually age quicker when NOT being driven. They warm up from driving, and this helps the rubber stay "lubricated". They can dry out when they just sit. This is why when I buy tires, I tell the tire place that I know how to read the manufacture date stamped on the tire, and I will not accept any tire that is over 1.5 years old.
