I agree with tobe....
Tires should last 5-6 years...When an old tire blows, we simply say it
was old and weathercracked.... That happens to the sidewalls.  Even a
tire with lots of tread may be old...To find the build date look on the
inside side of the tire you will find a DOT followed by three sets of
numbers. The third set will have 4 digits these are week of the year
and year. 5004 is the 50th week of 2004.
When you buy new tires, be sure they are new... When I had my oil
changed a couple of months ago, I was told my tires were beginning to
crack.  The guy had some tires on sale at a very low price.  When I
checked the date of manufacture, I found they were only 3 months newer
than my 3 years old I am currently running...
One reason we do not hear much about UV damage on auto tires is that
they usually wear out first..15,000 miles per year, which isn't much in
today's world, will mean that your tires need replacing every 4 years,
just before the UV damage begins to show....
If you are running 6 year old tires, please be sure they are properly
inflated and not overloaded.  The tires on my trailer are rated at 1875
lbs each at 50 psi...I need to be sure I am not towing at 2000 lbs per
tire...Always check the tire pressure, don't forget the spare before
every outing... and on long trips every three days.. check the lug nuts
while you're down there with the tire pressure gauge....I once towed on
a 4000 miler which lasted 14 days... never had a problem with lug nuts
nor tire pressure, but the peace of mind was worth the small amout of
time it took to check every 3rd day.
Leon in Texas

as long as you don't allow ventilation from the outside to the inside of
the tire, you're OK  :-)

Mel