Fact #1: When a tire is driven, the pressure and flexing motion circulate the internal oils through the rubber. These oils lubricate the internal rubber and keep it from drying and stiffening. So tires that are used less are often more vulnerable to aging effects.
Fact #2: Oxidation of rubber occurs much faster under high heat than low heat.
Fact #3: During rubber manufacturing process, each passenger tire require 5 gallons of butadiene and styrene oil and another 2 gallons of oil to generate the energy to manufacture said tire. For a total of 7 gallons of oil, commercial truck tires require 22 gallons of oil.
Like most of you, my TT spare tire is on my back bumper, wrapped in a black tire cover and receives direct sunlight most of the day. I recently replaced my TT's five tires and the unused spare tire looked remarkably worse than the 4 used tires. Hmmm, guess there is some truth in Fact #2.
IMO, if my spare tire was included in a tire rotation plan, the spare tire would age slower.
I have joined the rotate tire camp and we'll see what happens. Each spring I remove my tires and grease the wheel bearings and inspect the brakes. So, starting next spring my plan is to rotate TT tires using the "rearward cross" rotation pattern. Gotta take the tire off to grease the bearings, so why not? No harm, no foul.