Actually that's the funny part, and it never even occurred to me until now. In the original movie from '77, Obi-Wan never says he trained Anakin (that part should've been more obvious to me) but more importantly he never says Anakin was younger than him. It's just that it's a conclusion that can be drawn when you consider that Owen knew Anakin and Owen calls Ben Kenobi "a crazy old man." Even in Return of the Jedi when we find out Vader and Anakin are the same person, Obi-Wan never explicitly states that Anakin was younger than him, although he did say in the original movie that Vader was a "young jedi." It's just that with the line "when I first knew him, you're father was already a great pilot" you can easily construe that Obi-Wan meant Anakin was exceptionally skilled for his young (as in younger than Obi-Wan's) age. Another possible interpretation is that Obi-Wan says "already a great pilot" because he didn't meet Anakin until he was a great pilot. As for Shaw being much older than Guinness in real life, if you just look at it in terms of the movies it's easy to justify since Vader/Anakin would seem that old after all the lava pit damage he sustained. Nevertheless, that is a failing of the prequels in my opinion. I remember one of the first rumors about Episode III was that we would see Anakin/Vader gradually collecting the pieces that make up his armor. I thought this was a cool idea because it would make the movie a nice bridge to Episode IV, and I was expecting the entire movie to be that, not just the last 5 fracking minutes.