Well, there are people that are still kickin a year before they die. Yoda has lived almost a millenia. Why do you assume that an alien ages exactly the same as a human?
Because; Luke was a valuable asset to both of them, and their rise to ultimate power.
Anakin was still a youngan, think about how you will be when you're 50. I know I'll probably be different. It wasn't that Anakin was stupid, he was a sucker for his emotions. That's one of the traps of the dark side, giving in to your emotions.
Oh, and nothing personal, but for some reason stealing someone's midichlorions sounds like the dumbest idea I've ever heard of for SW.