Board Thread:General Discussion/@comment-31283382-20150404011045/@comment-24317446-20150404050844

The fact that Central 46 had to sentence him to 20,000 years in Muken suggests two things. The first is they can't simply execute him, which means he's unkillable. The second is that they sentenced him to 20,000 years of imprisonment. That's almost ten times Yamamoto's lifespan. He definitely has to have eternal life as well.

Being completely unable to die has some symbolism as well. As we know, the role of the Soul King is to be the linchpin of existence, and help transfer souls between Soul Society and the World of the Living in an endless loop. However, because Aizen is completely immortal, he can't participate in this cycle, which means, in a sense, the Soul King can never comprehend Aizen, as Aizen is outside of his experience.