𝐒𝐎𝐔𝐋 𝐑𝐄𝐏𝐑𝐎𝐃𝐔𝐂𝐓𝐈𝐎𝐍 & 𝐀𝐆𝐈𝐍𝐆
Childbirth for a human and childbirth for a soul are two very different things. As we’ve seen in canon, the children in Rukongai are almost always souls that died as children and crossed over and thus the families in Rukongai are a hodgepodge of unrelated souls. It is possible for a soul to become ‘pregnant’ and give birth, however, instead of simply creating a flesh and blood body like a human would, they’re creating an entirely new soul.
Creating souls is about as difficult as it sounds (and this is by no means the only way it’s done: see Nemu). Both parents have to have a substantial amount of reiryoku in order to form a new soul. This is why you don’t often see anyone in Rukongai giving birth, as the mass majority of them are average souls with lifespans very akin to a living one. They age and die at the same rate as a living human and thus keep a sort of ‘auto-balance’ of souls coming and going in Soul Society.
With all that in mind, getting pregnant is no easy feat and still a fairly uncommon thing to see (but certainly not unheard of) even inside of Seireitei. It’s still mostly left to nobility to go through the arduous task of creating heirs, as most shinigami don’t really think to actively try and have families of their own. With powerful parents, it’s very likely the created soul will also have copious amounts of reiryoku as well, which leaves a very large question mark as to when that new soul will ever come to experience a physical human life.
The aging process itself also does not seem to be the same for all souls, however, it does appear that children do age at a normal rate (with very few exceptions). At what age this process begins to slow seems to be dependent on the soul. Ichika, for example, is ten years old and appears to be physically that age. Toshiro, as another example, notes that within a five year span he had literally not aged at all in any way (not even his hair grew). That would suggest that the amount of reiryoku the new soul has plays heavily in how their body progresses, but also that a zanpakuto spirit can heavily influence their physical growth as well (I believe good examples of this is Unohana, Toshiro, and possibly even Yamamoto).
A basic generalized idea one can gather from canon is that it takes no more than fifty years for a soul to reach physical adulthood. Of course, there are always glaring inconsistencies in Kubo’s writing, as in, the fact that Nanao would have been a child for at least five hundred years ... so it’s all to be taken with a grain of salt and up to personal interpretation.