User blog comment:God of nothing/What are Shinigami?/@comment-1387259-20130922185713

Alot of details concerning Shinigami haven't been addressed in the ways your questions go. We know they are souls, and given basic statistics, it's reasonable to believe that there are many people who do die at a young age (about 16 people die on the planet every 10 seconds in real life, there's bound to be children in that group at times). But given that Kubo can do what he wants with his own universe, we can assume he decided to make Soul Society, while technically an afterlife, a whole separate dimension, where aging and mating are possible.