Board Thread:Questions and Answers/@comment-23924105-20140225204908/@comment-30945510-20171130055313

Well, in the first or second season the captains were talking with Tōshirō Hitsugaya, and told him that if he did not get proper sleep, he was never going to age, and remain a child. I'm not sure if this was just friendly teasing between Captain's, but if that's how it apparently works, their aging system works the exact opposite of human aging. Meaning that if they do not get good sleep, they are blessed with youth...?

But like you said, this can even be a shotty explanation. Kuchiki could probably be explained away because she's a lieutenant to the man of 1000 expressions. So needless to say she doesn't sleep ever. Tōshirō Hitsugaya is way too serious about everything, so yeah. But Uruhara, he kind of seems like one of those lazy good-for-nothing guys that sleeps a lot, but he doesn't really seem to have aged either from season 10 in the past, to the present day. And the head Captain's just forever old.

So, I think they do age, it's just that with the lack of need for sleep, none of them really choose to sleep anyways, so they don't age. But on top of that, they are immortal. But that's my Theory.