James Gunn just confirmed Batman won't be middle-aged, meaning Batman will be early-to-mid 30s at most when we see first him in the DCU.
I still think we're seeing different points in time to introduce us to these characters in the DCU from a specific framing point, some of this stuff might be set in like 2015 or 2010 - I think it's been said the films tell the history of these characters non-linearly so they standalone as strong films by themselves.
That said, I could see a 19 year old Bruce training with 19 year old Talia al Ghul and they conceive Damian. It doesn't have to be post-return to Gotham.