If you don’t know spiritual development then it’s good to have a basic grasp of at least one of the theories. There are lots of spiritual development ideas many tied into the theories of cognitive thought, the whole relm of abstract thinking and self awareness. Generally the most common spiritual development scale used is that of Fowler, with Piaget being the loudest voice of child development and Gardener on intelligence’s.
I could try and summarise here but there are three nice summaries available through usefulcharts.com so I suggest you have a read through one there.
WHY?? well because when we see the way our age group sees the world and God it helps us to give them better answers to their questions, chose better activities for their learning, and understand why they react positively or negatively to what we do.