6 year old tires are nothing. My truck is a 1991 F-150. 31 years old with over 160,000 miles on it. A few years back I bought a new set of tires for it. (The 4th set including the tires that came on the car when it was delivered new). I had never touched the spare, except to add air every once and a while.
When Discount sold me the last set of 4, they pulled the spare out from under the truck, and the guy couldn't believe it was the same one the truck was delivered with. He told me the tire was dry rotted, and was totally unsafe. I said as long as it holds air, I'm good with it. He argued with me, and being the cheap SOB I am, I didn't budge.
In desperation, he finally GAVE ME A NEW 5th tire FOR FREE, as a spare replacement! He said he couldn't in good conscience let me leave with that crappy, rotten old spare. I told him that it still had the little rubber nipples on the tread, because it had never been mounted. He just smiled at me, shaking his head as he walked away.