After retiring with 40 years in the tire industry, I can tell you the big deal on age that has exploded in the last few years is MARKETER DRIVEN. It is like the 'use by' dates on cereal, milk, and everything else you buy. I see a mattress marketer now stating mattresses should be changed every 8 years ! ! ! !
Best guide is use the info provided by most major tire MANUFACTURERS, not MARKETERS. You will find a big difference in their recommendations.
IMHO J-ROOSTER's post above is the best information I see so far in this post.