James Gunn debunked the latest "middle aged casting" rumor, though, I wouldn't be surprised if the actor does end up being in his mid 30s to late 30s when he's cast.
I just don't think Gunn is looking for a new Batman right now .
35 to 40 would be fine imo, since this Batman is supposed to be old enough to, at least, have a 10 year old son .
Most of the actors they've cast to play Batman in the feature films have been in their 30s anyway , so, that's more than old enough to be an " established " Batman whose been operating for at least a decade.