For starters, what kind of comparisons are we talking about? Schwarzenegger is now 65 years old, returning to the Terminator series 10 years after his last appearance, Terminator 3, was released (2003). Aging actor, resurrection of a franchise, portraying an action type hero?
Bruce Willis took up the role of John McClane in Live Free or Die Hard (2007) at age 52, with the movie coming 12 years after the previous Die Hard. Hmm, that's pretty much younger than Arnold -- Willis just looks old, jeez, and he has forever. But again, the movie actually did pretty well, being reviewed as, and I'm paraphrasing, "pretty ridiculous, but also pretty awesome." It was again generally perceived as better than the prior installment. Score 2 for the comebackers.
For the release of Indiana Jones and the Kingdom of the Crystal Skull (2008), Harrison Ford was 66 and that film came fully 19 years after the third installment. Hey, pretty close, right?! There is one little bit of a difference here, though. Crystal Skull was kind of set up to give a reboot of the series with Shia LeBeouf, and Terminator already played this card. Remember Terminator Salvation (you know, the one with Christian Bale?) released recently in 2009? Yeah, that film did not do so well. Unfortunately for our comparison, Crystal Skull was also generally considered the weakest film of the franchise so far. Still, consensus was the movie was well put together, just kind of "meh" and what you would expect. Not bad, not great, just a little disappointing given what came before.
All right, so so far we are thinking Terminator 5 has a shot to be good. Terminator is probably not quite as well loved as Indiana Jones, but it's got fans. Arnold is also a little bit old for this kind of stuff, even compared to his peers, but he's not completely crazy and really, has there ever been a guy in better shape? The thing is, Ford and Willis are good actors (Sorry Sly, but come on, The Expendables, Cobra, Over the Top?! Let's just say you and Wasted Cinema are on a first name basis). And Arnold? You mean this Arnold?