Anakin was Obi-Wan’s padawan. It’s natural that he would be younger than him, and not necessarily his “contemporary.”
It always struck me more as a brotherhood rather than a father/son relationship, so the 16 year age difference in the prequels always strained credulity for me. But having an 82 year-old play someone who is supposed to be 45 is really stretching it, even granted the terrible deformities and rough life.
What would make the prequels and the OT play nice is if there was a 24 year gap between the PT and OT.
Alec Guinness was 62 in ANH. With a canonical age of 38 in ROTS, this makes him perfectly cast in terms of age, and is more in keeping with the ‘old fossil’ talk in the movie.
Mark Hamill was 25 in ANH. This de-ages him by only one year.
Carrie Fisher was 20 in ANH. This ages her by 4 years, but since she acts older than her age anyway, it’s quite believable.
Phil Brown (Owen) was 60 in ANH. With Joel Edgerton in ROTS being 30, he would be de-aged by 6 years. The years have not been kind for him.
Shelagh Fraser (Beru) was 56 in ANH. With Bonnie Piesse being 22 in ROTS, she would be de-aged by 10 years. The years really haven’t been kind.
Sebastian Shaw was 82 in ROTJ. Hayden Christensen was 25 in ROTS (canonically 23), this would make him 51-53 in ROTJ, a de-aging of about 30 years. It’s an extreme aging, but at least he’s beyond middle age at this point.
It’s actually a 19 year gap between Episodes III and IV. Close enough. One logical explanation for the fast aging of Owen, Beru and Obi-Wan during that time is simply the fact that they have been living on a harsh desert planet with two suns. There’s also the matter of stress, with the thought of the Empire knocking on your door at any time.