Then why haven't DC brought him to middle age? Because once they state he's in his forties, then fans of other Bat-family characters will be bemoaning why he or she can't take Bruce's place. They would have a point, too, because those younger superheroes would haver bodies in better shape to take on crime in Gotham. There's even a precedent for it - the Batman of Earth-2 was semi-retired by the time he turned fifty, while Dick and Helena took over.
Bruce could be Batman well beyond middle age, IMO, and be enjoyable to read. But he wouldn't be the best of the Bat family long before that happened. It would be illogical to think Dick, Tim, Jason, Damien, etc. couldn't kick his butt by then and would be unfair to fans of those characters, IMO.