At 41 years of age, it seems likely that the Manchester By The Sea star could only convincingly play an older and more seasoned Dick Grayson a.k.a Nightwing.
Hypothetically speaking, even if Casey Affleck does eventually decide to sign on to play the role of Robin, we can pretty much guarantee that it would not be a traditional depiction of the character.
That said, the DCEU has made plenty of changes to the comic book lore (in Affleck’s words, “they mix it up a lot”) so anything is possible.
At his core, Robin is designed from the ground up to be a young character that kids and younger members of the audience can identify with on a more personal level.