In the world of Bleach, some Soul Eaters age faster than others. But why?
The Bleach franchise is full of inconsistencies that are often graciously overlooked by the dedicated fanbase. The confusing ages of the Soul Society’s inhabitants is one of them. Some thousand-year-old Soul Eaters look worn and full of wrinkles, while others look as fresh as ever despite being their contemporaries. So what is the secret of their beauty?
According to one fan theory, the aging process in the Shinigami world is based on nothing more than one’s mental age. Your motivations, objectives, zest for life — they all determine your looks. If you feel old, you look old, it’s as simple as that. Most souls confined within the walls of Seireitei do not stagnate, but grow; thus, they only mature with time, although at different rates.
This theory would explain the appearance of Captain Commander Yamamoto, who is presented as a world-weary character with seemingly no ambitions other than protecting his subordinates, and Captain Unohana, a heroine that has yet to achieve her true secret goal. Arguably, Unohana’s youthful appearance could also be explained by the simple fact that she’s a woman, and female characters in general are rarely allowed to age.
A more practical explanation would be that the aging process depends on one’s spiritual power. Basically, the stronger a Shinigami is, the slower they age. The more you stay at the age that matches your potential, being in your prime longer while you're at your personal peak.
This fan theory is backed up by the fact that Renji and Rukia went from being children to young adults on the streets of the Soul Society, and pretty much stopped aging right after they got accepted into the Shinigami Academy. Captain Hitsugaya, on the other hand, is a child prodigy that demonstrated immense potential from a very young age, so it makes sense that he still looks like a child.
Of course, the only person who knows with absolute certainty how aging works in Bleach is the creator himself. Tite Kubo likes to occasionally throw the curious fans a bone and reveal some of the secrets of the Bleach world. Perhaps, one day he’ll clarify this mystery for us as well.
Source: Reddit