Board Thread:General Discussion/@comment-30881771-20180827142148/@comment-25065646-20180827214426

Essentially, "they age when the plot needs them to, and if not, they don't". It is a very vague area of Bleach lore because it's handled somewhat inconsistently, IIRC initially it was implied that Souls of departed humans do not age at all due to them being already dead, but then we've had Souls dying and being born within the Soul Society. Plus pretty much everyone who is able to age does so at a rate unique to their own, e.g. all the Vizards, Kisuke & Yoruichi looking exactly the same as one hundred years ago, Genryusai aging from biologically around 40 to something 80 or 90 over the course of one millenium, and then there's the Royal Guard with let's say Nimaiya looking no more than 30 and actually having been alive longer as Genryusai. Personally, I like to believe somethign along the lines of: The greater one's Spiritual Power -> longer lifespan/slower aging process, or Greater Spiritual Power -> more control over how old you look/how fast you age