My understanding of it is that we all need to go through child development which includes emotional development. If you don't get to emotionally develop or miss some child development, that part of you will still be stuck at that age. Traumas mess with child development and emotional development. I got to adulthood at about a 5 year old with emotional development. I simply had not learned certain things/didn't get to experience certain things that would have helped me to develop. Thankfully I was able to pick up where I left off in my 30's and now I'm closer to my actual age.

This is a continuum and it goes all the way to Dissociative Identity Disorder.