I have a theory has to how they’d get away with Schwarzenegger looking old for Terminator Genesis. It’s not a great theory, but it makes some sense.
Okay, the Terminators have living organic skin, this skin bleeds, heals, sweats, and have bad breath. If this skin is living and organic, it’s possible that maybe, the skin can also age. The idea is, it’s not the Terminator itself that is old, it’s just the skin. The Terminators live for a long time, but the skin probably doesn’t, it ages like a human and after a long time, the skin would probably age so long it would rot.
But Skynet would probably update his Terminators. But we’re assuming that the Terminator in this is a “good” Terminator, reprogramed by the Resistance, so the Resistance probably wouldn’t have access to the technology or priorities to update the Terminator. The humans are in a war against machines, do you really think they care whether there Terminator soldier looks young? So the Terminator in this probably fought with the Resistance for a few years and his skin aged before before being sent back in time.
That’s just my theory. Could be wrong, I don’t know, don’t care. Just a fun little thing to contemplate.