It seems to me from some experience with low/no use cars, that tires are good until around the 13-14 year mark. That's like, cars sitting in a garage, unused after some number of years.
Beyond about that, the sidewall cracking and overall condition of the rubber seems to go south.
Id suspect that's why the Japanese (and I think American guidelines when talking date of manufacture) is 10 years (IIRC, American guideline is 6 years in service). It gives a 30-50% factor of safety on top of whatever intrinsic design toughness exists.
Id run them so long as they look OK, but Id be saving for another set at minimum at the 10 year mark, and not let them stay in use beyond 10-12 or so.
Given that its a truck, if the low mileage is because it pulls a travel trailer or boat or something, then the story may be vastly different...