In a recent interview, James Gunn clarified that Supergirl is canonically younger than Superman in the DCU. He says that approaching the question from this angle is "assuming that everybody on Krypton is the same,

The obvious assumption is that he's directing a direct sequel to this year's "Superman." Indeed, Gunn has already indicated he's writing the next DC film to star David Corenswet as the iconic Man of Steel.