oh_riginal said:
But there is still no explanation why Obi Wan aged so badly compared to Qui Gon and Dooku (within character, I don't mean the actors).

BUT... Mark Hamill aged pretty badly didn't he? Imagine him playing Luke nowadays....

Luke: "Ben, why didn't you tell me?? ... Why didn't you tell me! Why didn't you tell me how to age better!"

Obi Wan's Ghost: "Hey don't look at me, my master, Qui Gon, didn't pass on that knowledge to me! That bastard!"

I believe that I can explain that. For 19 years, Luke and Obi-Wan lived on Tattooine, a desert planet with two suns. My theory is that those who are not native to Tattooine may experience some excessive aging over a period of years due to the two sun. And as we know from the prequels, Luke and Obi-Wan were not born on Tattooine.