Stephen,
How lucky do you feel
Used tires are at best a dice roll.
Deterioriation can take place internally due to aging, temperature and/or impact.
Cracks can appear in the surface, but can also grow unseen in the interior, as rubber compounds are "alive," that is, changing over time. Tires are vulcanized at the point of manufacture but this does not necessarily freeze their physical and chemical structure at that point. Further changes will occur as they age, even in ideal storage conditions. Adhesion of the rubber to the embedded tire cord, or between the layers of rubber can change and weaken over time.
If tires are to be used as "show car only" where the vehicle isn't driven any appreciable distance and then only relatively slowly, using elderly tires is likely OK. If the tires see use on the street or on the track that's another thing entirely.
Tires over about six years old are suspect. This is not to say they will inexorably fail, but the odds of a failure due to age-related internal weakness does increase. Some track-oriented organizations limit the tires age to three years or less.
The mode of failure in elderly tires can be anything from a slow leak to a sudden scattering of the structure over the landscape.
So, are you feeling lucky?
LOL
Tom