Ford now recommends that tires be replaced after six years because the
materials do oxidize and deteriorate over time (causing the cracky dry
rot look).

The reason you hear so much talk about "aging tires" these days is
because of the Firestone fiasco.  Now everyone is trying to cover
their a.s by telling people to buy new tires every 6 years.  There is
no downside from the tire industry/auto industry standpoint to feed
this bunk since it just puts more money in their pockets when people
replace perfectly good tires.  I've driven on tires over 10 years old
at high speed with no problems many times.  In fact, the only tires I
have ever had problems with have been relatively new ones and usually
they have been Firestones.  In my personal experience all tire
failures are from manufacturing defects, not "ageing".  Of course if
the tire has been UNmounted and laying in the dirt and getting rained
on for 8 years that's a whole different story but I don't think you
are talking about that kind of old tire.