Board Thread:Questions and Answers/@comment-26592105-20150708010654/@comment-11051098-20150708172633

To be fair, Yamamoto founded the Shin'o academy 2100 years ago, and may have been close to 2500 years old when he died. Unohana could have been 300-500 years old when Yama-jii created the Gotei 13, leaving her at maybe 60% of his age. Still, as per Xilinoc's point, if we assume Unohana and Yama-jii were roughly 1500 and 2500 years old when they died, she did look way younger and he way older than one would assume, compared to each other.

One could also assume that the soul age thing works like human age. Some humans look older than they are and some look way younger than they are. That could very well be the case with Unohana and Yama-jii.