|Don't be fooled by good looking older tires, whatever the tread.|
Safety comes first. Replace old rubber.
With that said, I discovered the rear tire seemed in fine shape. Plenty of tread. Sidewalls are okay, I think. Then I found the tire was a puncture-proof Nimbus Armadillo, like the one I put on my Trek Antelope. I wondered. Should I consider reusing it on my own bike?
Thankfully, I researched further. Old rubber not only disintegrates, but becomes stiff. Inflexible tires do not corner well. I will heed others' advice: if you don't know the bike's history or suspect it's been in storage, why take a chance? Do yourself a favor, replace the rubber. Period.