As tyres do not have a definitive "Do Not Use After" date, as consumers we have to be guided by legislation, and our own abilities to check and maintain them.
As we do not normally have method to check the internal structure there is always going to be some guesswork involved, and age might be rough indicator. But as we have seen it's far from a definitive criteria. We have to keep in mind, tyres will deteriorate, and an older tyre has an increasing risk of age related failure. And that the rate of deterioration is affected by the materials used in the tyre, and how the tyre is manufactured and where the tyre is stored and used.
With so many uncontrolled variables no one can possibly say exactly how old a tyre will be before a it should be considered unsafe to use.
As with a number of well intentioned caravaning advice the industry puts forward, there is little if any hard evidence of how the advice was formulated, and wether it's still relevant or necessary, but in the case of caravan tyres, five to seven years seems to be sensibly cautious.
However regardless of the industry advise, it is very important that every driver should regularly check their tyres and get any tyre that is demonstrating pressure drop, insufficient tread depth, bulges, cracking or cuts to the chords at least checked repaired or replaced.