Children are, by nature, narcissistic. They do not have the life experience to determine the "other". There are specific developmental stages relating to "normal" ages at which children understand the concept of other's needs, consience, etc. I am not a developmental psychologist, but I have read a little, as a parent and a teacher. I think that early on, children see themselves as the center of the universe. Little by little they learn that this is not true. In adolescence, (the second toddlerhood!!) most people are becoming who they will be in terms of who they present to the world (don't go by how they are with parents!!!). However, I think the person with a narcissitic personality disorder, or at least the one I know currently giving me such crap....has an arrested adolescence! They never get beyond that. They function well enough to pass for normal, so "intervention" is impossible to determine a need for, but their lack of consience, or ability to empathize is frozen in the MEMEME track of the young child/adolescent.
I am sure there is a lot out there about this.
Listen, I see kids who act just horribly toward others as kindergarteners, who by 4th grade are just wonderful people. Then again, I see some who are never going to change, because the parents are NEVER going to let them know when they are wrong. "A**holes in training", I call them. Parental values that simply don't include being a good person. Denial as a parent creates some pretty bad behaving people. Then again, I've seen GREAT parenting...all to no avail, with a whacked out kid... but it's more the former than the latter, IMO..
There just doesn't seem to be a one size fits all answer in this life, is there???