I got the tyres changed on my Delta today,not because they were short of tread, but because I didn't know how old they were, and was worried that the rubber would be perishing. I ordered a set of Uniroyal Rallye tyres to be fitted at a local place (the tyres recommended to me if I wanted grip with a low mileage car).
Well, as I was packing up to leave (having had my car admired by at least two people who came in, one of the guys working on changing the tyres came and showed me one of my old tyres. "I didn't think the tyres needed changing" he said, before pointing out to me the splits in the side walls only revealed by pressing the side of the tyre when deflated. "I'm suprised that hasn't gone bang already" he reassured me, in the manner of Black Country mechanics.
So the story is, tyres wear out over time, not just due to mileage. I was very pleased that my hunch to change them had been proved right. And as a bonus, my car handles and rides better, and has far lighter steering. Just a little bit of encouragement for anyone wondering whether its time to buy new rubber...