I think a Batman in his 40's maybe 50's could be interesting. We saw a little bit in BVS and JL, but is a movie only with an older Batman be a bigger risk then with a younger Batman?
Right, we saw a little bit of that, and Affleck (mid 40s) is already hanging up the cape. So, there goes that franchise, and here we are... softly rebooting(ish) with a new Batman. There are physical demands being an action hero that a 40 or 50 year old might not want to take on. Not everyone is Tom Cruise or Harrison Ford.
So, you have longevity of an actor in the role for your franchise...
And, yeah, I do think a younger actor, in his early 30s, would not only be more believable as Batman, but, also get more people in the audience. How many people want to watch Batman at the END of his career?
There is Logan, that's a counter example, but, that's more about Hugh Jackman playing Wolverine and saying good bye to a role. And, as popular as Wolverine was in comic book circles, it took Hugh Jackman to put him into the pop culture at large. Batman was already there. Everyone knew Batman even if they had never read a comic or seen a movie. He was ubiquitous. Batman is bigger than the actor. (tee hee, in Wolverine's case, the actor was, indeed, bigger than the character.)
So, yeah, I don't think it makes great business sense to do an older Batman story, unless it's a one off. I would watch it. I would rather see a bunch of different Batman stories, different kinds of Batman stories. But, then, I'm not running a studio.