Obi Wan was probably early 20's in Phantom Menace, Anakin Skywalker was like 9 years old.
Obi Wan would probaly have been late 20's to early 30's by the time of episode II.
Question is how long did the Clone Wars last, judging from how much Anakin seemed to have aged, I'd say 5 to 10 years. That could actually put Obi Wan into his 40's by the time of Episode III.
So we could be seeing Obi Wan in his 60s, bear in mind that it could simply be that Obi Wan's hair went grey early and he was younger than he looked.
according to lore he was 57 in ANH
he also lived as a hermit in a desert for 19 years... that environment isn't exactly conducive to keeping a youthful look