Board Thread:Questions and Answers/@comment-23924105-20140225204908/@comment-24282510-20140226023355

Not sure on this.

I think it was said in the manga that only those with spirit power could age and they were also the only ones that needed to eat or they would die of starvation. And those without did not.

So that in itself begins up so many questions. That we will never know because the time for those answers have pasted i.e. be out of storyline in the manga.