How do souls age in Soul Society? How do souls age in Soul Society?  


   People cannot die of old age resulting in the Captain Commander keeping his position for a thousand years.


There are some souls who look like kids and some who are adults.  In the manga, the makers flashbacked into the past of the Captian Commander showing that he had black hair instead of looking like what he was then.  Also, when Ichigo left the Precipice World he aged what looked like a couple years.

So how do souls age in Bleach?
The only explanation I can find is on bleach.wikia.com, which says:


  One never gets hungry (if she or he doesn't have spiritual powers) and aging is slowed to a great extent, with lifespans of 2000 or more years not being unheard of, though such ages are limited to Shinigami or other Soul Society dwellers with high spiritual power.


There is also an explanation on http://tvtropes.org/pmwiki/pmwiki.php/Headscratchers/BleachSoulSociety that essentially says that it is really inconsistent within the show and that there are a number of different theories, including that characters age inversely proportional to the amount of spiritual power that they use and that at some point aging just stops.

To summarize, there doesn't seem to be a clear explanation in-universe, but the aging is slower than usual, with much longer life-spans than regular humans.