The age difference between Obi-Wan and Anakin

In the original Star Wars, Darth Vader calls Obi-Wan an old man. Then Anakin is played by the older Sebastian Shaw in ROTJ. Since the 2004 Special Edition DVDs, Shaw has been digitally taken out and replaced by the younger Hayden Christensen and as such you may argue that this change was done for the better. Although there is still the fact that when you get to the prequels, there isn't that much of an age difference between Obi-Wan and Anakin (Ewan McGregor is only ten years older than Christensen in real life). I suppose that using Shaw in '83 was problematic in the first place for the old man comment in ANH, so I'm not sure which movie really is the most to blame.