Obi-Wan's on going age

Im sure this has been asked before, but considering the time line of the prequels, why is ob-wan so old in a new hope. So he is around 24 in TPM, 34 in ATC and 37 in RTS. And the time gap between episode 3 and 4 is 19 years, so that would make him only 56, yet he looks like he is in his late 60, the whole reason for him and Vader having that slow saber fight is because ones half machine and the others an old man. 56, wasn't qui-gon around that age in episode 1, and he was pretty lively, i really don't want to sound like a fan boy looking so deep into all these aspects, but i have been very interested in that, what do you guys think?

p.s. Vader would have also been around 42 at the time of episode 4, just an interesting thought, not much older then Han

EDIT: Welcome to the boards! Sorry that your first post is a lock, but we have a thread already just below this one about the ages of the characters!

http://boards.theforce.net/the_star_wars_saga/b10456/30810809/p1/?60