Short careers?

Bob Pastor's career spanned only seven years.
Billy Conn's career lasted 14 but the last 6 years included a 4 year break due to the war and he only had one "real" fight which was his rematch against Louis. Conn achieved everything that is notable about him as a fighter during 1934-1942, about eight years.

 

The one that comes to mind was a 6'9 heavyweight prospect named "Taz Titan" who got featured on an undercard of Tuesday Night Fights and looked pretty imposing. He had a car accident and never fought again.

Joe Lipsey cut things short and retired after losing by KO to Bernard Hopkins and went into ministry I believe.

 

Earlier this year I watched an old Sot Chitalda fight and he was defending against a Japanese kid named Hideaki Kamishiro. The Japanese kid, one could see, had talent and won the first round on my card. But Chitalda went into another gear and, despite the kid showing heart and trying his best, Kamishiro's corner retired him after 7 rounds. Still, I felt the kid was a very bright prospect and looked him up to see how he got on in his career. As it turns out that was his last fight. Yep, all 8 of them. They ran their kid into a title fight with Sot Chitalda after only 7 fights. What a shame to ruin a good prospect who, brought along properly, could have had a long and successful career.