Financially, the decline kinda started with FFIX, but I'm guessing you're talking about quality here.

imo, the decline started with FFXII. FFXII and FFXIII wasn't magical like every other Final Fantasy title, and I don't think me getting older is the reason why I think this. Sure FFXIII was great compared to FFXII, and I enjoyed it well enough, but it doesn't have FFV to FFX beat.