I think Fuzz Townshend has been calling for a 10 year limit to be applied in law, recently? Amusingly, my car is 29 this year. Last year, I had the spare wheel out (it's never been fitted!) and was marvelling at the "original equipment" Pirelli on it. Just for curiosity, I put it on the car and went for a drive. It felt remarkably "normal" but had noticeably less grip than the others (although, the others weren't the same make and model so it wasn't a very scientific comparison)! It certainly squealed like mad when getting near the limit of adhesion!
Something stingy within me really abhors the idea of binning a tyre with full tread on it (even the little moulding knobbles were still there!) so I've started a policy of including my spare in the rotation now, meaning that it will take its share of wear in future. (Not the 28 year old one, the new one that's now on it)!