Age in comic books is something of a moving target. Some characters are perpetually a certain age (Comics Batman has been about 34 or 35 for most comics fans' lives), while others age at a weird pace (some of the once-young X-men are in their 30s now; it only took about five decades for them to age 18ish years). The age of our movie superheroes is much more set in stone, but that actually presents a bit of a problem when it comes to a Robin movie.
For example, if the movie does end up being a prequel, it would likely feature the most iconic Robin that comics fans know: the plucky teenage sidekick. This means a major multimillion-dollar tentpole would be headlined by a teenage boy. It would basically be Spy Kids but with a cape, which is a tough sell for any audience, and even tougher for the more mature audience Warner Bros. and DC have tried to cultivate with movies like Suicide Squad. The movie could feature an older Robin, such as a pre-death Jason Todd, but then you run into the other age problem of Batman and Robin being too close in age (just look at the forgettable Batman Forever, in which Val Kilmer and Chris O'Donnell come off more as bickering brothers than a father-figure and his adopted son).
Why not just have Bruce Wayne adopt a new sidekick in the present day? Well, Batman is established as being in his mid-40s, and it may come off as odd for him to adopt someone a quarter of his age, knowing that his last sidekick died a violent death at the hands of criminals who are still at large.