Board Thread:General Discussion/@comment-4620155-20130410100635/@comment-4666155-20130410230238

Isshin seems to have lost his powers, I would assume this affected his slower aging in some manner, and, given that normal humans can see him, being in a gigai must also affect his aging and speed it up to that of a normal human, though this is just spitballing.

And again, everyone seems to age differently. Unohana seems to be closest in age to Yama-jii, and she doesn't look old at all.