Here's set of tyres that's taken to limits on track. I buy lots of take-offs from MotoAm racers @ Sears Point Sonoma. Stocked up because I was moving away. Here's last set of Alpha-13 tyres I spooned on last year. It was already 6-yrs old at time:
Again, I don't care about age, but inspect them carefully for cracks. None that I could find, so I gradually took them to limit. By 3rd session, I had found that limit and started to go over ever so slightly and overcooked them a little bit. Little balls on edges are created by tyre sliding and scrubbing off top-layer, which collects on edge where there's no longer contact.
Grip was amazing! Was able to lean all way to edges and slide them around. No problems with grip. This is about 3-4x faster than I would ever go on streets.
of tyre is ultimate test indicator. It encompasses entire past history of tyre: previous use, proper inflations and cleaning, storage and temperature control. Age is irrelevant.
Modern silica-based tyres (starting around early-'90s) are simply amazing compared to their carbon-black predecessors. The additional poly-vinyl cross-linking of silica compounds makes them much more resistant to heat-cycling and UV/ozone damage. Even performance tyres like Supercorsa TD or Metzeler TD have more grip than outright racing slicks from couple decades ago. And they don't require warmers and don't degrade with each heat-cycle either.