I'm 39, and terrified of doctors and medical stuff. Always have been. My dad was too. I know now that I will get old and sick and die, and that is hard for me. I'm trying hard to adjust my perspectives, but "knowing" for 38 years that I'd never die is hard to shake and I bought in completely.
Death is hard to accept, as others mentioned above that ceasing to exist is a mind bender. That's why religions exist. They fill people with thoughts of another, even better existence. I always thought that the whole eternal nap thing for those that didn't make it in the new system was terrifying on some level. Now knowing that it is the only eventuality is tough.
I wish I was taught the cycle of life and appreciated it from a young age. I also wish 38 years wasn't wasted in a damn cult.