User blog comment:Xilinoc/Ch. 631 - friend/@comment-8669536-20150618120459/@comment-7805464-20150620040129

Yeah there is a Kishi-Reishi conversion machine Urahara used back then on Soul Society arc. And possibly Yhwach's old army did this too using some kind of techinque? To become souls and go to SS.

Since they're still souls, living in the shadows of Seiretei, so they also age like souls, just like the other Soul Society inhabitants. Different to the Ishidas who live in human bodies.