Board Thread:Questions and Answers/@comment-24963288-20190412183516/@comment-39511866-20190518225434

It might be proportional to the amount of spirit energy an individual has. When Isshin leaves Soul Society I think 35 years ago, he looks 20 something, and when Kisuke seals him in the human-gigai, he ages to look like 40 something when Ichigo is 15. We know during this time, before Ichigo awakened Zangetsu/Shiro, Isshin's powers were still sealed, and so pretty much aged like a human, despite being a soul in a gigai. The only problem with this idea is that Yamamoto seems MUCH stronger that Yachiru Unohana, yet she physically ages less.