I know that you guys are going to want to smack me here, but I started feeling old when I turned fourteen, and it just got worse every birthday after that.
I read a LOT of fantasy books when I was younger, and the kids in those were always 12-13. Then there were the 16 year olds. Then the 18 year olds. And after that, you were too old to ever have a magical adventure again, and your life was over.
I did not believe in magic, but I was still crushed when I turned fourteen and nothing had happened and I had to start high school like all the other boring kids.