Did Obi-wan Age Normally?
At the end of Revenge of the Sith, Obi-wan Kenobi, played by Ewan MacGregor, is present for the births of Luke and Leia, and he appears to be approximately thirty-five years old, give or take. When we again see Obi-wan, he is quite an older man, perhaps in his late sixties or early seventies, played by Sir Alec Guinness. Yet Luke, if the reading I've done is correct, is approximately nineteen years old at the beginning of A New Hope. If this is incorrect, by all means correct me, but if it's true, why would Obi-wan have aged so much?
Using the force, like the addict that he is, ages you immensely. Take a look at Yoda, a man in his mid forties, who looks like he has been around for 800 years.
For the record, Obi-wan was 57 at his death. Not young, but, as you say, not as old as he looks.
@Kevin And 57 is -exactly- the right time frame (3 years off) for Slytherincess's estimate of him being ~35 at Luke's birth, and Luke being ~19 in _A New Hope_.
Alec Guinness was 63 when Star Wars was released in 1977 (and would have been slightly younger during filming). Ewan McGregor was 34 when Episode III was released. If we assume each actor portrayed a character of exactly his own age, that implies a gap of 29 years -- which is close to Mark Hamill's age of 26 when Episode IV was released. Apparently both Alec Guinness and Mark Hamill were portraying characters significantly younger than they were.
Sir Alec Guinness was 61-62 during principal filming of Star Wars. The timeline has Obi-Wan as 57. That's not a huge discrepancy.
Don't forget the old Obi-Wan had a full beard. Maybe he'll look his age if he shaved (just look at Episode I and II-III)
No, Obi-Wan looked exactly right for his age in A New Hope. It was the fumbling idiocy of an ego soaked moron poorly casting A listers to boost box office sales instead of making a good film that created the illusion of an age gap. I just chalk up ep 1-3 as a dream in some drunkard's mind and refuse to accept that they exist.
In-Universe, no real canon answer I know of, but he was a man living in fairly poor conditions (backwater crap of a planet, with really poor infrastructure, and he lived in a cave in a boonies to boot), who had major psychological blows (Leaving aside any war-related PTSD, he raised the damned Darth Vader! And everything he ever held dear was destroyed, along with most of his friends).
Oh, and he has responsibility to watch over the pretty much only hope of the Galaxy, and is fully stymied in raising him properly due to Owen Lars' intransigence.
Yeppers. Living in a barren wasteland desert is hell on the skin. Dry winds, heat, immense amounts of UV Rays (especially considering the whole "two suns" thing). All of these things prematurely age a person. And imagine what the relative lack of life forms to feed his Force jones would be like to him. I'd also imagine that he works pretty hard to get by. Even finding water to drink would be difficult.
Also to add to that......I know someone personally who is 27 and looks like 47! and he is a normal guy living in a big city. So looking far older than one is can happen to anybody