At what age does the awareness of mortality begin?

We just returned from a wonderful, but brief trip to the Netherlands to see my dad marry his wonderful new wife, Adrienne. Emy caught a cold either in the Netherlands or shortly after we got back, and so was pretty stuffed up. Last night Emy and I were chatting while getting her tucked in for sleep. She said, "Daddy, I'm scared to fall asleep." I asked her why and she said, "I'm worried that if I can't breathe through my nose and I forget to breathe through my mouth I'll die in the night!"

Wow. She's never said anything of the kind to me so far. Does this mean her innocence is slowly disappearing? Is she going to become overly cautious? Can you really forget to breathe? What should I say?

I said, "Don't worry sweetie! Your body won't forget to breathe. You'll be breathing out your mouth just fine when your body realizes it can't get enough air through your nose. You know, like when you're running really fast, or riding your bike - have you ever noticed you start breathing though your mouth without even thinking about it?"

She said, "Oh yeah, that's true. OK, thanks Daddy!" Big hug.

Just like that. She even thanked me for providing some relief to her anxiety.

So I guess fear of death definitely starts by age six.