It seems especially odd to me that people worry about the age of a Superman actor. Like what, are people thinking that Old Man Kent would be a weak old man who can't keep up? Are they worried about the actor not being able to maintain the physicality? Because I'm struggling to see why anyone would think either thing; Clark's often written as becoming *more* powerful as he ages, and plenty of current day superhero actors are over 40 and they look fine. Hollywood pays for diet specialists and personal trainers; that 50 year old superhero actor is probably in better shape than any of us (and I'm in pretty damn good shape for the shape I'm in!). Not to mention makeup, good lighting, a little cgi glow-up....
If it's an origin story that's one thing. But otherwise? Cast whoever is the best actor and looks the part.
And I wouldn't even worry about "actor longevity" at all. Not with DC. Marvel, yeah that's a consideration because you're looking at potentially a decade of movies. But DC barely manages to get one film off the ground, struggle even more to make that one film successful and sequel-worthy, and the last thing they should be thinking is "can this actor still play the role in 2032?"