It happens sometimes below Enlightenment Age. I remember when starting my second account encountering Enlightenment, Industrials, and Globals in Bronze Age. It was pretty awesome if they happened to have any unguarded farms and/or caravans actually.
Even now with that account in gunpowder age I'll stumble across IA or GA bases a few times a week. It doesn't seem like they can revenge me if I attack though, or maybe I'm not worth the bother for them.
Not sure what the cause may be though. And my IA account has yet to stumble across any Atomic Age bases.