I'd make myself 5 years younger so I can do things that I should have done but didn't do (like studying or taking more seriously my health issues). When I get again to be 20 years old, I'll make myself inmortal. Then, I'll keep my powers, but I wouldn't use them for big things. I'd spend my time travelling and learning new languages. And when the world becomes unbearable, then I'll die.
Well. I guess I'd be a vampire xD but without the blood thirst.