The bases of death anxiety... which affect you the most?

[True Federalist (진정한 연방 주의자)]

Oh wow, really? That's fascinating. I had no idea the brain had much left to develop after 15-16 or so.

While I can't speak specifically to the neurology of mortality awareness, the brain does remain fairly plastic and adventurous until around 25 or so.  It's one reason why teenagers make so many boneheaded decisions.  It's evolution's way of helping the species as a whole, since some of those leaps of faith teenagers make prove to be advantageous despite there being no rational way of knowing ahead of time.

[True Federalist (진정한 연방 주의자)]

I love discussing death!

And yet you wonder why some people don't like you. 

More seriously, no matter one's belief system, death is not the eradication of self, as even in a severely materialistic POV, it doesn't change the fact that a period of time you existed and will always exist in that time frame.