I've noticed that the Vaizard have not aged since they have become hollowfied. I was just rereading the entire series so far and i was near the end of reading the chapters about the lives of the Vaizard.
For example Akon from the Research and Development Institute. He's been part of it since it began which is around the time before the Hollowfication of the Vaizard. He was a child then, around the same age as Hiyori. After 101 years Hiyori is still looks the same age and Akon is already an adult... Can someone please explain why this is so?