Board Thread:Questions and Answers/@comment-26592105-20150708010654/@comment-5398287-20150708071938

Honestly, it's all based on stress. The more a Shinigami stresses, the faster they age. That's why Unohana looks like she's 25 and Yamamoto looks like he's 125 despite them being the same age; Unohana's the chillest gal on the planet, while Yamamoto runs an entire military.