Obi-Wan, as Old Ben, was only 57 when he was killed by Vader on the Death Star in the year 0 BBY. Of course, he became a Force Spirit and so is immortal in some sense. Although it hasn't been officially addressed, the general opinion is that he looks much older because of his time scavenging in the Tatooine desert. More significantly, he had to deal with the mental anguish of what his former Padawan had done.