The people responsible for all the Terminator sequels have never put a lot of effort into hiding the fact Schwarzenegger is no longer the 30-something he was when he made the first film. This is not that noticeable in T2, but I just saw the Genisys trailer and he really does look every day his 67 years. I find this jarring because the T800 we briefly see at the end of Salvation, fresh out of the Terminator factory, looks exactly as Schwarzenegger did back in 1984.
The only way I can think of reconciling these differences is by capitalizing on the fact that T800s are covered in living tissue, which does age. So, the Terminator in The Terminator looked young because he was sent to the past just as he walked out of the Terminator factory. The one in T2 had already been in service a few years in the future, so his tissue cover already looked aged by the time he was sent to the past, and similarly for the one in T3. The one in Genisys looks the oldest because he might have been in service for God knows how many years in the future, and then another few decades between Sarah Connor's infancy and the present. Plenty of time for the tissue cover to naturally develop the wrinkles that Schwarzenegger has!
Is there any in-universe explanation for the age differences?