But then that are those that think FF12 fits in just fine in the dark age category....

Depending on the size of groups you want to break it up into I would place the golden age at about FF4-FF9 or FF4-FF7. For me the 4-7 stretch constitutes the 'core' of final fantasy.

Honestly to me it seems like they just divided the series up into three chronological groups and then added names. I don't know how you could not place a division between Square pre-merger and pre-Spirits Within and the SE that came out of it.