When was the last time you saw a horror anime-loving 12 year old cosplayer?
Never seen one?
Now you have.
My first cosplay was when I dressed up as a Powerpuff Girl when I was 6. Wait, does that even count? xD
OK, my first PROPER cosplay was when I dressed up as a Cyberman at a Doctor Who convention.
I believe that Halloween is just an excuse to cosplay in public. Which, of course, it is.