For me, it was the realization that one of your parents were dying right in front of your eyes and that everyone was hiding it from you because you were young. My dad was sick with stage four cancer and muscle atrophy so his condition deteriorated over a couple of months. He refused treatment (for some reason he believed he was a bad person? I never understood this)
I was about 12/13 years old when I realized very soon, a man who I looked up to, used to swing me around and drive like a healthy person, was now incapable of sitting up without assistance, couldn’t go to the bathroom himself, couldn’t brush his own teeth and made my mom and I stay up with him until dawn because he was afraid of dying in his sleep.
My family left me in the dark about this because I was young. I always joked with my dad how he was going to see me graduated from middle school, and go to a really good high school and college. Realizing that he was never going to see me graduate and that I was going to have to say goodbye one day hit like a truck.