I see a lot of fans say Bleach ages for shinigami are inconsistent but that's not the case. The high amount of reiatsu you have the the more your body develops as time goes by. You have to remember they are spiritual beings not mortal beings like humans are. The majority of pluses in the Rukongai don't have a lot of reiatsu so they rarely age at all whether they are young or old. This is right is an example:
Also another thing is when you die in the world of the living they don't reincarnate into different people or anything. Whatever age or time period they died in the world of the living they stay that way until they reincarnated back to the world of living as a different person all together that's how the cycle works just for a regular spirits. Now for shinigami from average(seated officers) and above captain depending on the high amount reiatsu they have is what determines their growth that includes mental state as well. Also Yamamoto and unohana are not the same age 1000 years so of plus Yamamoto has live for more millenniums than unohana has in her lifetime so of course she didn't age as much as he did. Is shinigami has a limit of a certain amount of reiatsu they can handle especially in hitsugaya case that his bankai(full) forces him to age in order to handle the power that he normally couldn't wield. Aizen wouldn't have this problem because was much older than toshiro plus his reiatsu is twice a day of a normal captain that this wouldn't even strain his body.
Also another thing is when you die in the world of the living they don't reincarnate into different people or anything. Whatever age or time period they died in the world of the living they stay that way until they reincarnated back to the world of living as a different person all together that's how the cycle works just for a regular spirits. Now for shinigami from average(seated officers) and above captain depending on the high amount reiatsu they have is what determines their growth that includes mental state as well. Also Yamamoto and unohana are not the same age 1000 years so of plus Yamamoto has live for more millenniums than unohana has in her lifetime so of course she didn't age as much as he did. Is shinigami has a limit of a certain amount of reiatsu they can handle especially in hitsugaya case that his bankai(full) forces him to age in order to handle the power that he normally couldn't wield. Aizen wouldn't have this problem because was much older than toshiro plus his reiatsu is twice a day of a normal captain that this wouldn't even strain his body.