Even though Terminator creator James Cameron is not directly involved in the new film Terminator Genisys, he did give them some pretty plot-pivotal advice—specifically about how this new film could incorporate a 67-year-old Arnold Schwarzenegger into the new Terminator franchise. And this explains a lot. Spoilers.
This weekend at the LA Egyptian Theatre's celebration of of the 30th anniversary of The Terminator, Cameron let this little quote slip to Deadline, greatly hinting that Arnold is indeed, not playing a human.
"I pointed out that the outer covering (of the Terminator) was actually not synthetic, that it was organic and therefore could age. You could theoretically have a Terminator that was sent back in time, missed his target, and ended up just kind of living on in society. Because he is a learning computer and has a brain as a central processor he could actually become more human as he went along without getting discovered."
This was allegedly the tip Cameron gave to producer David Ellison. So now we know that the figure we've seen from leaked set pics is an aged Terminator, not the original model for the Terminator or the Terminator inventor. This quote also implies that this version of the Terminator has been living in the background for years. There was a Terminator functioning in the real world before May 12, 1984 (when the first Terminator was sent back in the original Terminator movie). This means A LOT of things. It means we could see a Terminator pulling strings in the '70s with a younger Sarah Connor. It means we could see an older Terminator version fight a younger Terminator. Really anything is possible now that there are multiple Terminators populating the planet back in time. Interesting, very interesting.