Board Thread:Questions and Answers/@comment-3001900-20131024200959/@comment-4992373-20131026185641

Probably has to do with what Sei said but add in spiritual power as well. The only reason I say this is actually because of "Tenza Zangetsu" -> Ywach.

When in the Inner World, he is shown to be younger and this is because the Bankai state keeping the spiritual pressure focused, rather then it dispersing like most other bankai. Spiritual power has also shown to strengthen upon ones resolve so...

1) As years go by, possibly regrets come to mind and not as much spiritual energy returns to the user, thus causing aging but still at a much slower rate.

2) Kenpachi was a Kid but had a huge amount of spiritual energy, so it may be that he is actually older then what we have been shown. When Kenpachi felt regret about not being able to fight Unohana again, he suppressed a ton of his spiritual energy, and with that regret he got older. Eventually though he found the thrill in fights again and so he now maintains his 30~40 year old age apperance.

Thats my take on it though.