It makes it so accessing empathy (and judgement in general) non-instantaneous for teens. Adults have the mylenation, so can far more quickly access the prefrontal cortex's gifts in a split second manner.
http://hrweb.mit.edu/worklife/youngadult/brain.html for more on this effect
While you certainly may be able to learn things as well, this is a developmental brain function issue too. There is a limited amount of training, and a certain amount of waiting, involved in getting an adult.
Wikipedia article on Empathy: "most children do not show a fully fledged theory of mind until around the age of four"
There isn't much you can do to "teach" it.
http://hrweb.mit.edu/worklife/youngadult/brain.html for more
I've always thought of myself as empathetic, but was surprised when I became even more so after a life changing event. I've spoken to many with a similar experience.
Large, traumatic events can bring the world into focus and shift perspective for those who experience them. Perhaps it's not an increase in empathy, but rather a better understanding of what really matters to me. I don't yell at the customer service rep on the phone solely because I care about their feelings, but mostly because it's simply not that big a deal in the grand scheme of things whether my internet is out or if I miss my flight. If someone I care about is in danger, I doubt I would be as "empathetic".
There's always the old adage: "You catch more flies with honey"