Board Thread:Questions and Answers/@comment-3001900-20131024200959/@comment-2089817-20131025190140

I've always believed that the physical age may have to do with something akin to the eight stages of development or something, where one ages physically depending on the experiences and changes that the soul itself has undergone.

If we were to assume Yamamoto is around 2500-3000 years old, considering how he looked at the age of 1500-2000, he looked like he could pass for Kyoraku's age. But then suddenly, another couple hundred years, while Kyoraku and Ukitake were in the Academy, he suddenly becomes an old geezer. I assume it could be because the Gotei 13 now entered a time of peace and his soul was now at peace, in contrast to the blood-thirsty warhawk mode that he was in during the prior times, up until the Quincy Blood War.

Unohana probably did not go through much emotional or spiritual change since her meeting with Zaraki, so it could be why it seems she hasn't aged a day.

Byakuya seemingly aged immediately, after a hundred years, from teenager to adult in his presumed "30's", possibly due to the mourning of his wife's passing and having to conceal away his emotional feelings in order to uphold the law, something that was deep down eating away at him. He was born in the peaceful times so he probably has a far less amount of tolerance for the pain of loss, unlike those such as Yamamoto and Kyoraku that were born and raised during warring times.

Though that's just my guess.