I wasn't happy about an "older Batman" when they announced it, but let's just look at what's on screen; this "past his prime" Batman is WAY more physically capable and acrobatic that ANY Batman we've seen before in live action, only comparable to animated and game form.
I think the whole "aged" aspect was overblown as a selling point, we know he's mid 40s because we're aware of every little detail and interviews and everything the filmmakers said, but I think in the actual movies they'll keep his age kinda vague (like with RDJ's Tony Stark) as in early-mid 40s, since Affleck can easily pass for younger than his age; I mean, he looked a lot younger than what we were told in the movie.
Like how in the trailers he said "I'm getting slow in my old age, Alfred", but the actual movie line was "You're getting slow in your old age, Alfred"; it was the selling point, and as Affleck himself said in an interview, that was a Batman at the end of his rope (mentally and emotionally), but in JL we see a Batman "reborn" so to speak.
We don't have an "old" Batman, we have an experienced one.