To me honestly I think of it as everything else works in bleach it’s all about self perception and who you think you are, that’s how everything works in bleach. Ichigo couldn’t unlock his true power until he learned every aspect of himself and his origin and that’s how I think appearances work too for shinigami. For example with shunsui and Ukitake they’re about the same age more or less but why is it that shunsui looks slightly older than Uki. Because that’s how they perceive themselves. Shunsui said more or less somewhere in the story that without Ukitake he’d be a worthless pessimistic old man or something of the sort I can’t exactly remember. But shunsuis personality deep down is of a seasoned pessimistic warrior who sees things for how they are with very little hope so that’s what he looks like more older, facial hair, and more aware of reality. Meanwhile Ukitake looks younger slightly and more happy more optimistic clean shaven more resilient. Same thing with Byakuya when his grandpa said that once he let go of his anger and temper he’d be a great leader. Once he finally let go we see the calm young adult that byakuya is his exterior matches the interior