CHARGE!!!! ^.^ said:
Nobodies have been around scince heartless were around. The org. came into being when Xehanort turned into a heartless, with the other Ansem apprentinces.
And Heartless were created when Xehanort and his apprentices did experiments
(think about it, how come suddenly people would start losing their hearts? How could anyone know unless they studied it?). At first, it corrupted Radiant Garden
(which is the history of Hollow Bastion, etc.) before starting to spread out into the worlds, and invading them slowly.
Unless I read wrong, of course.
The Org. XIII weren't around by the timespan of ten years before. Xehanort wasn't even found by Ansem The Wise yet, so there's no way the first six members were created, thus, no 13th Order whatsoever.
Xigbar only knew because he's been around, and has actually seen them, or even met them probably
(I believe he'll appear as his Somebody in the next game. Why hint that he saw them if it wasn't going to get anywhere?) Nobodies remember their memories, though we never really hear them speak about it, except for Xemnas when talking to Ansem The Wise and, you know, Xigbar. The special case was Roxas, since Sora didn't last long as a Heartless and couldn't retain all the memories.
Just because Xehanort has an X in his name, doesn't mean he's a Nobody
(so if a girl was name Trixie, she will be a Nobody too?). And is only for Org. members, which, again, weren't even around at that time.