Board Thread:Questions and Answers/@comment-24963288-20190412183516/@comment-5398287-20190412195019

By all accounts souls "grow up" at a fairly normal rate when they're "young" (aka below 20) then once they hit their 20s it just slows down hard. It also just varies person to person like in real life - Yamamoto and Unohana were both around 1000 years ago, yet one is a wrinkly old man while the other looks 30 at most.