He's 38 in Episode III. Which would make him 57 in Episode IV. In these movies, Ewin and Alec were 34 and 63, respectively. So basically, over the course of 19 years, Ewin would have to age about 29 years in order to look like Alec, or in other words he would have to age 33% faster than normal. I find that believable because my father went from looking like he was in his early-20's in his late 30's to looking like he was in his late 60's when he was in his early 50's. So basically, in about 15 years he looked like he aged 30 years. Age has not treated that man too well. And he wasn't even living on a desert planet. So all things considered, I actually think that Obi-Wan aged with quite a bit of grace, given his circumstances.

Oh, and I kind of hope that the future Obi-Wan movie is a Western. And that it has the Force music. And that it's directed by James Mangold. The possibly still alive Ennio Morricone as composer? I'd respect that. Al Silvestri wouldn't be bad, either.