Yeah, that would be a bad direction to take, imo, because its a very untrue or inauthentic take on the Bruce/Talia/Ra's relationship. Ra's training up Bruce was established in one movie: Batman Begins. And even people who like the Christopher Nolan Batman films would point out that that take on Ra's isn't the best. And then because of that fact, Bruce and Ra's come to blows then very early in his career and therefore instead of Batman and Talia having a genuine love story, Marion Cotillard's Talia is on a mission of revenge and merely seduces Bruce to advance that mission.
Also, and this is the more important part: Ra's and Talia don't respect and/or love Bruce. They have those feelings for Batman. The whole thing that makes that dynamic between the three of them fascinating is because Ra's is impressed with Batman's skill and prowess as Batman and sees him as a worthy challenger/suitor for his daughter. There's a reason that both of them more often than not call him "The Detective" as opposed to "Bruce." And that wouldn't be the same if Ra's himself had trained him up. One is a student/master dynamic. The other is a meeting of equals.
And yes, there are people who would say "but Batman is Bruce. Bruce Wayne is the mask." The "Bruce" part of Bruce Wayne is still a part of who he is. Talia may still love "Bruce" but she definitely loves "Batman" more. So introducing Talia to "Bruce" instead of to "Batman" kind of diminishes that whole relationship.
Also, as if mid to late 30s is ancient. LOL. Zachary Levi and Chris Evans are both in their 40s and they are in better shape than I am despite me being about a decade younger than them. And I'm a guy who stays fit.