Jack the Ripper. He was on quite a tear and just walked away for some reason.
Guess we should mention Jon-Benet Ramsey.
That seems like an odd time to return.
That is SO evil, Thanks for the laugh. 
see post #6
Richard Farina.I have a hunch that had he lived, Bob Dylan would be but a musical footnote.
Bruce Lee and 20 years later his son Brandon
Phoebe Cates, whom Kevin Kline has kept all to himself.
I’m pretty sure he didn’t make a splash at all, if you believe the New Testament…
My vote is for Janis Joplin, but it’s probably for the best. I’d hate to see her on the casino circuit.
Charmian Carr, who played Liesl in The Sound of Music, the oldest daughter and the most prominent. It was her first movie, but she got her own song (“Sixteen Going on Seventeen”). She was nominated for a Laurel award as one of the great “new faces.”
It was also her last movie—at age 22. Then she got married, made babies, started an interior decorating business, and that was that.
Grace Kelly started her acting carreer at age 20, then at age 26 became a princess and never acted again.
Glyndwr Michael, the star of Operation Mincemeat. 
Grigori Perelman. While he did more than just one thing, he became famous for proving the Poincare Conjecture, and then quit mathematics.
Not to forget:
Lee Harvey Oswald
Sarah Holcomb was in Animal House and Caddyshack. Rumor has it she got into drugs and quit acting to get clean.
This movie is based on her life
Cincinatus seems like a good example. (Wikipedia quote: " When his son was convicted and condemned to death, Cincinnatus was forced to live in humble circumstances, working on his own small farm, until an invasion caused him to be called to serve Rome as dictator, an office which he immediately resigned after completing his task of defeating the rivaling tribes of the Aequians, Sabines, and Volscians. ")
Possibly Alexander the Great would count?
Jack Batty, didn’t Anne Frank become famous after she died?
I definitely think it counts. He was a bankable actor when he walked away.
He shows up on “Where are they now?” lists and he’s one of very few actors on such lists for whom the answer is “where he’d rather be.”
Yeah, Janis in Branson is not a pretty picture.