Can XII really be considered to have been part of FF's golden age? It seems more like the start of the series' current era to me, given its troubled development history and general divisiveness (though I'll admit that I'm a bit biased, given my own issues with the game >_>; ).
According to my experience with the fandom "how far the golden age of FF goes" tends to be divided on these groups:
- Last game under Hironobu Sakaguchi's supervision (FF9)
- Last game under the Squaresoft brand (FF10)
- Last game before Tetsuya Nomura became "influential" in the series (FF6)
There are others but in my view and experience these three are the most common division.
I think I'd divide FF into these periods.
First Period; Final Fantasy 1 to 3, the series' genesis period when the basics were being fleshed out.
Second Period; Final Fantasy 4 to 6, the time when everything was flowing well and the series established more of its current identity.
Third Period; Final Fantasy 7 to 9, the peak of the series popularity and market dominance.
Fourth Period; FF 10 to 12, mainly dominated by spin-off projects such as the Compilation and basically when FF full expanded beyond the main series.
Fifth Period; FF 13 to 15, a time of strife and reorganisation compounded by development problems and a changing gaming landscape.
Sixth Period; We're in it now. And who knows what comes next.
I'd like to add to this that XIV's use of Crystal Tools (totally unsuitable engine for MMO or even open world development without extensive overhauling) didn't help matters.