Wanted to post this during the outage:

We have 3 human characters who live through the period between 3 and 4: Obi-Wan, Beru, and Owen. They all live on Tatooine and they all age terribly. So the simple answer is that the twin suns make you look older than you are.

I was going to say that maybe people look older in the SW Universe, but TFA takes place an appropriate amount of time after RotJ for the actors to be as old as they are. But if any of them live on Tatooine in the meantime, my theory goes right out.