A few years ago there was a mechanic who fitted some New Old Stock tyres to his MGB, he had the equipment to fit them himself and being a mechanic, inspected them for any visible defects, inside and out, after a few hundred miles whilst driving on the M56 one of the tyres collapsed, the car flipped he was killed. The Police determined that although the tyres were "as new" the age of the tyre was the problem, in this case I believe they were 20 years old.
Frankly I don't care that that legally I can run 5 year old tyres on my trailers and caravan, for the same reason I don't have 5 year old brake fluid or engine oil, yes it will probably work but why take the risk, the cost over 5 years to replace tyres is minimal when you look at all the other running and depreciation costs.