Board Thread:Questions and Answers/@comment-24463723-20140126011146/@comment-8986886-20140126023339

Well, Rukia became a Shinigami around 51 years ago. According to Byakuya, that's when she was adopted into the Kuchiki family, and Renji must have followed a couple of years later, since they joined the Academy together. I guess the more pieces you discover, the easier it gets to find the next missing piece. You'll have to re-read alot of chapters to be able to solve the puzzle. ^^