Isn't it impossible for him to become one of the thirteen darknesses? I mean didn't in DDD it said he had to travel back in time so the event could unfold and him become Master Xehanort. Could YX and MX even exist in the same time frame? So why do people think he is one of the thirteen darkness?

I dunno.

Fool. Because he can and he will and cause it's Nomura!

He has to travel back in time after the deed is done I guess. That's the only way he will become Master Xehanort and start the thing all over again. 

Young Xehanort did travel back in time so he can lead Sora and Riku in The World That Never Was. Master Xehanort told him what the future looks like and he gave his younger self a task to make Riku their 13th member but Riku became immune to darkness so they did what the keyblade did and move down the list, which it was Sora and Sora became their main target. But their plans had failed so we don't know who will be the 13th member. And Young Xehanort is one of the official 13 seekers of darkness, he's the 12th member and besides did you see him sitting on one of the those tall chairs? Sitting in those tall chairs pretty much confirms that he is a official member.

Edited May 28, 201411 yr by Movies798

Nothing involving time travel is set in stone, and any rules that have been established will be broken in a moment's notice.

He could return for the final battle and be sent back after his defeat. I think that's what's going to happen to all the Darknesses. Because death (well, violent death at least) doesn't exist in KH, I think that after their defeat they'll be sent back to their original time zones and loose the ability to travel, barring them from ever becoming a problem again as each meets their own fate in their own time.

Edited May 28, 201411 yr by Isamu_Kuno

Isn't it impossible for him to become one of the thirteen darknesses? I mean didn't in DDD it said he had to travel back in time so the event could unfold and him become Master Xehanort. Could YX and MX even exist in the same time frame? So why do people think he is one of the thirteen darkness?

Young Xehanort is the base of a time loop. Master Xehanort ensured the events by forcing his past self to follow the same path he did. Because Ansem went back in time a version of him will always exist to send Young Xehanort to the future everytime he returns to the past.

Isn't it impossible for him to become one of the thirteen darknesses? I mean didn't in DDD it said he had to travel back in time so the event could unfold and him become Master Xehanort. Could YX and MX even exist in the same time frame? So why do people think he is one of the thirteen darkness?

Xehanort can do ANYTHING.