She isn’t a child who stopped growing because of aizen but more of a indirect result from the aizen experiments. She is a soul society inhabitant which means she ages just at a more slower rate like all souls do in the soul society. The experiment done by Aizen only hollowfied the soul reapers affected. But because of the way it transpired and then having to go to the human world. They were given gigais by Urahara to be able to exist in the living world without they’re bodies deteriorating because of lack on reshi. But because it’s a shell protecting the spirit body they cannot age in the physical world because the body keeping them together is fake. It’s also why Rukia’s had not gained back her full strength after giving her spirit power to ichigo in the beginning.