If the tyre is more than 5 years old, don't use it!
Why not?
The make and then condition of a tyre are far more important than just age as a factor not you use it. UK law as of Feb 21 tyres more than 10yrs old can't be used on the front steered axles of HGVs, buses, coaches or all single wheels fitted to a minibus (9 to 16 passenger seats).
This doesn't apply to cars or bikes etc, this has all come about due to the market being flooded with cheap/crap Far East budget tyres. They're cheap for a reason, the compounds they use are unstable and will be effected by atmospheric conditions alone, UV etc even when just stored. They either split or go hard, so hard they no longer grip although the tyre looks to be in good condition.
Yes most tyre manufactures and some bigger tyre fitting companies Kwik Fit for one will advise not more than 5, why to sell more tyres! I've just had tyres replaced for a customer, that were dated 02 fitted to a GSX-R1000, they were fine, in all respects (dry & wet) just worn to legal limit. The new tyres fitted ae date 15, tyres can be in the system for over 5yrs before they're fitted, especially bikes tyres and not so common sizes of other tyres, hence why the government limit is 10yrs, which allows for that.
If this tyre is 20yrs old, but if made by a quality brand, stored properly and unused, the condition of the tyre is the most important factor not it's age.
The reason behind this legislation for HGV's, PSV's is quite simple. When the tyres used on these vehicles get to (or below) the legal tread depth, they are "re-cut" usually in the owners own workshop, with no other inspection to the condition of the carcass.
It's straightforward really - rubber degrades (you wouldn't want to ride on a 20 year old Durex would you?
[size=78%] ). [/size]In the aviation industry, there are regulations up the ying yang for the storage of the little beasts. Maybe the same should be applied to bike tyres as well.
Personally, I change mine, irrespective of wear, every two years at the very most. Takes into account the delay in getting from the mould to the rim.