In 2017 an old friend bought a used Norfolk Gypsy & took it to the builders in Norfolk to have it overhauled.
They also overhauled the trailer & fitted three tyres to it - two on the road wheels & the other as a spare.
On his way back to Cornwall he stopped overnight in Leicester with me.
Both road wheel tyres were heavily cracked, the spare looked perfect - even the moulding wispy bits were still on the treads of all three.
Changed the worst one for the spare.
3 figure dates on the tyres so pre 2000 - I don't have the exact date to hand.
2 tyres failed on his way back to Cornwall & the boat & trailer finished the trip on a low loader to a garage near St Austell. The breakdown service he's a member of didn't want to know as the fault wasn't with the car so he had to foot the bill.*
Old tyres may look fine but they simply aren't worth the risk.
*Worth checking the Ts&Cs of yours if you tow as it could come as a very unpleasant surprise.