Back in the day tantalizing hints were encountered of a D&D world, present not just in the adventure modules but as little snippets of lore in places like the artifact descriptions in the Dungeon Master's Guide. The Ring of Gaxx, dread Vecna with his Hand and Eye, the Wind Dukes of Aaqa... these were things that had history, clearly. But only much, much later were these items developed in any concrete way. Until then they were mysterious rumors to spark the imagination.
From the standpoint of writing and designing An Age Undreamed Of, I'm taking the approach that there will be a strongly implied setting present in the game, and strictly that. While AAUO will be set in a mythical Forsaken Age, and while there will be names sprinkled throughout the text to suggest the history and cultures of this age, there will be no "official" version of that setting ever presented.
On the other hand, I do have a vast number of setting ideas and even a fairly fully realized world in the pipeline, which I will plan to get back to working on once the AAUO rules are done. At least one of these, the Sundered Reach, is a sandbox wilderness that I plan to develop for AAUO. The words "forsaken age" will likely appear together in the text of that work. It will be one small corner of a world with that appellation. But even if the book provides glimpses of a Forsaken Age, your Forsaken Age will be completely different.
I'm a big believer in strong setting materials; I did, after all, spend years working on exactly that. But I think an old-school RPG needs evocation and creative freedom. Even where there is a framework in place, as with the old World of Greyhawk folio, the world is never really presented to you. Instead, within some skeleton the world developed organically in play at each table. A setting defined down to the smallest parameter like the Forgotten Realms, neat and lovingly developed as it is, does lose the unique fingerprint stamped upon it by the group.
Don't get me wrong, though - I admire the Forgotten Realms as a piece of fine craftsmanship, and many fine campaigns have been set there. Too, the original FR boxed set is, I think, brilliantly evocative without being confining in the least. I'd say you could make a good case for that item being among the finest presentations of an old school setting. And even more, I admire masterpieces like Hârn and Tékumel. But what those two were, and what the Forgotten Realms became, are really a different find of framework than AAUO is going for.
(Today's graphic is a map I did a little while ago of a Forsaken Age. It may or may not see use in anything like its present form, but it is the landscape where the Sundered Reach will be set.)