I dunno. It is something we have talked about at home, and as far as I can tell, dh, dd and I share the trait that we developed some distance and had independent internal lives early on. By early on I mean that we all remember this as part of who we were as far back as we can remember. Not sure how common this is.
I think it's after adolescence. Until the frontal lobe (which handles reasoning, planning, and insight) is completely developed, we have a brain that is very dependent on the external world for our ideas and attitudes on many things.
Ok I'll play.