Board Thread:Questions and Answers/@comment-7397924-20130626142244/@comment-8986886-20130626170156

Xilinoc wrote: Seeing as how Unohana is at least 1000 years old...

1000+ people have normal appearances, in the 2000+ they start to age.

Simple as that.

~My Theory~ When a Shinigami reaches 3000 years, they are forced to retire and are then placed in "The Old Geezers' Nest"; having become too embarrassing to their respective squads to be aloud to continue their duties. ^^