I contacted Metlzer about my over three old new tire and this was their response.
As for tire age our published statement from the catalog is as follows:
The mere passage of time (age) does not cause tires to deteriorate, but rather exposure
to outside forces. Such outside forces can include, but are not limited to: road hazards,
punctures, improper repairs, misalignment, under inflated operation, over inflated
operation, excessive heat caused by over deflected operation, excessive exposure to
ozone, improper storage conditions, etc. Tire companies can only have an impact on
a few of these exposures. Tire companies add antiozinants and anti-oxidants (antidegradation
compounds) to minimize degradation. Metzeler believes that since there
is no way to accurately predict what outside forces a tire will be exposed to there is
no scientifically supportable age limit that can be set for tires.