I think it likely made more sense in whatever the original context was long before the Prequels were a blip on anyone’s radar. Shaw’s performance is the key and he’s excellent in the little time he has onscreen. I’ve never considered his age before but he seems perfectly fine as a contemporary of Obi-Wan.
When you go back and think about watching that scene for the first time, he is a reasonable casting choice. When you think about what Obi-wan said in ANH, and early in ROTJ. Anakin was a student of Obi-wan’s. We aren’t really given the context that he should be younger, but that is implied, especially after we meet Yoda. Sebastian Shaw was older than Alec Guinness by 9 years. Hayden was younger than Ewan by 10 years. Can 25 years in that suit serving the dark side age someone that much? And let’s not forget the burn scars.
But then again look at Harrison Ford in TROS And Sebastian Shaw in ROTJ. Do they look the same age? They are, but do they look it? And who looked older in the ST, Hamill or Ford? And I think the makeup and costume kept Shaw from showing his true age. The images of him in the film as Darth and the images of him without makeup look very different.