I don't want to cheat, no.  and really don't want to use mods either.  was hoping there was a base-game setting that would let you start at, say, turn 50 or somehow after the first tech era, where you and the AI rivals have established empires already, kind of like being dropped into a star trek TNG era as the starting point of the game.

Yes, that would be a very useful option. Upvote from me.

Especially if combined with a changed Ideology situation matching the changed Age situation. As an example: Yes, you can start in the Age of War, but since you're there, you don't have any way of claiming the Pragmatic "Cannot Declare War on Me for 50 Turns" perk. After all, since you start the game with all those "early game" techs discovered and have the basic Military ones too, you shouldn't be at such a disadvantage if some race does declare War on you.

+1, I wonder how it would change early game strats

Cool!