Maybe it's just the desert climate.
Actually, just looking at Luke and Obi-Wan, if you take the ages of the actors as the ages of the characters, it is 26 years between III and VI, and in that 26 years Obi-Wan ages 29 years, so not that far off.
However, from his behaviour particularly, I always took Luke to be about 20, which makes the disparity closer to a decade.
Oddly enough, Leia seems more mature and could more believably be 26, but Carrie Fisher was 21 at the time, around the assumed age of the characters.
That put's the age disparity at 8 years.
It's John Connor all over again.