I don't think being in the "Golden Age" requires every game to necessarily be great. It's more about popularity and attitudes than anything. After VII, fans of the series were ridiculously excited to VIII. And even after VIII disappointed many, everyone was excited for IX. If FF had a Golden Age at all, those two games were probably in it.

Similarly, while XII was good, it was not part of that Golden Age. It was years in development, and a lot of enthusiasm for the series as a whole had waned at that time.