Is it? I'm glad it's an option, but I'm also glad retiring at 40 is an option (depending on a lot of stars aligning just right of course).
My answer to "how long do I want my career to be?" will never be "until my body or mind fail me".
Depends on how you think of it. Maybe I shouldn't have used the word "career" here, as I wrote code long before people paid me for it, and aim to keep doing it long after.
Depends on what you mean with 'career'. Brian Kernighan doesn't do this because he needs money, but because he would get bored otherwise.