What are your views on tyres which are 20 years old, regardless of mileage

Dried, cracked and dangerous.

Industry standard is replace at six years for safety.

Six years is wise but Michelin says up to 10 years if you inspect them. Different tire makers have different standards. Personally I have gone more than 12 on a collector car with Pirellis but it was ALWAYS garaged and the tires had low miles. 20 years? HELL NO!