[Engenia]

I read a report not long ago on a test done with exactly the same tyres manufactured some years (5?) apart.
The fresh set were installed and the rider cut a number of laps timing each. Then the stale set were installed and the session repeated. The first couple of laps were discounted as the rider gained confidence. The result was exactly the same times.
As far as I can understand, and from my own experience, once the thin oxidised layer is worn off, there is no difference old or young. Removing that layer takes only a handful of corners.
I'd be happy to use a tyre manufactured 7 or more years ago.
All bets are off if the rubber has cracked.

 

[Engenia]

A knobby has a vastly higher surface area than a road tyre, so when that surface oxidises/hardens, it can only be scrubbed off on the outside. The hardened surface of the walls remain harder and therefore stiffer. It's less compliant and won't grip as well.
A road tyre doesn't have that problem. It's tread shouldn't be moving.

 

[Engenia]

Lotus54 - True, trials tyres aren't as aggressively blocked as knobbies, but they do have far far more tread wall area than road tyres.
h3avyt - If you are still concerned, have a look at this -

- it bears out my experience.

 

[Engenia]

.. having seen the INSIDE of the casing that let go at 7 years old it was cracked up like safety glass,

Thanks Neil, that's definitely worth considering, especially if the tyre was stored in high humidity.
I've seen the corrosive effects on alloy wheels and engine casings of bikes that live on the coast. I'll now include steel belted tyres to that list.

However, it is easy enough to check for steel belts. Use a magnet.
Michelin Road 5 & Power RS are non-magnetic (so either kevlar or stainless steel & therefore less prone to corrosion),
but Metzler M9, Bridgestone BT016, Pirelli Superbike slick & Supercorsa are magnetic.

Also, if you mount your own tyres it's easy to check for internal damage. I do it without even thinking about it, but now I will pay even closer attention.