Some more age comparisons...
Shelagh Fraser, who played Aunt Beru, was 54 at the time of filming. Phil Brown, who played Uncle Owen, was 60. As seen in Episode III, Bonnie Piesse (Beru) was 21, and Joel Edgerton (Owen) was 30.
Being that they are 19 years between III and IV, you are looking at 40 years and 49 years, respectively. That's where the real age discrepancy is. Even with the reasonable aging effects of the environment, that's pushing it.