Re: TNG's "Too Short a Season" with Kirk...
The logic of the story's premise works better with a new character than it would have with Kirk. With a new character you can make him de-age to his 20s no problem by casting an actor in his 20s start off in horrible old age make up and be in no make up by the end of the episode. Had it been Kirk played by Shatner, he'd only be able to de-age to his 50s, and while rapidly de-aging from 80 or 90 to 50 could still leave the same result, there's greater impact with 90 to 20.
"Internet message boards aren't as funny today as they were ten years ago. I've stopped reading new posts." -The Simpsons 20th anniversary special.