Yes it's been established that he/his race originated in Zaterra but everything we know about him is a result of spending most of his time in Outworld. Probably hundreds if not thousands of years. I mean

To be honest.. i have no f...ing clue :D And does it really matters? I don't know.

Well.. I saw something interesting in the fight between Kotal and Raiden. He throws his sword against Raiden. I don't think i saw this move at the E3. This could mean we didn't even saw every Special move from the fighter at all :D Also he grabbles his opponent very often like King in the Tekken series. Hm... I loved playing with King in Tekken... so... :D

I think one of Kotal's variations will focus on grappling more so than the others. But yeah since variations seem to only add maybe two special moves we probably have seen most of them.

We actually don't know how old Reptile is.

What we DO know is that he didn't know about Zaterra or how his race went extinct until other characters like Khameleon and Nitara told him. All he knew was that if he pleased whatever badguy he's working for at the time like Kahn or Shinnok, they might find a way to rez them. (Note that Kahn and Shinnok are the ones who drove them extinct in the first place, proving how out of the loop Reptile is)

Which means he wasn't around when Zaterra was conquered, he was born in Outworld after it had already happened.

So there.

Not that any of this has to do with Kotal Kahn...

It doesn't mention it anywhere that he wasn't born when Zaterra went extinct. We only know that he just wasn't aware of HOW it went extinct. We know that Shao Kahn was responsible without Reptile knowing so he was loyal to him. He could very possibly be around when his realm went extinct, just not aware of who did it. We all know that Reptile is no genius. I just don't get why you guys are twisting small details up and trying to make it look like proof. There's no way we can know 100%.
Plus if we are talking about proof, Reptile only started showing signs of serious devolution in MKDA. Which means if he was born and raised in Outworld he should already be looking like a damn reptile by the 1st tournament not the 5th. That says that he was already an adult..umm Zaterran(??) when his home went extinct.
Anyway like I said before, everyone can have their own opinions. You have every right to believe he was born and raised in Outworld. I just don't see how. It's not like we're gonna agree on this anytime soon so let's just leave it here.

So have we established if Kotal is God of the sun yet?

Kotal Kahn : The sun will burn your soul.