I imagine the thought process when casting Shaw was “Dads are old, cast an old guy.” And that’s honestly the impression I get from Shaw Anakin, too. If you think about the actors’ ages, Shaw was way too old to play Luke’s dad. But in the final movie, he just gives me the vibe of the father of an adult son. So it never bothered me.
Maybe you could explain it away with how the dark side had been rotting him away for more than half his life.