I don't know when I realized. I already knew when I was in kindergarten. I would spend long hours staring through the chainlink fence out at the world and thinking how precious my time was and how i was wasting it. I don't know why I thought that at that age. I don't know why I didn't do more with my life if I thought that when I was so young. But I was still a child and came up with some weird ideas about how robots had no emotions and people did and people died so I'd suppress all my emotions and teachers got concerned I never smiled, which took some time to get over all that, even after I realized I was being ridiculous. People would ask me even into the sixth grade why I never smiled and I'd say "what's there to smile about?" perfectly matter of factly mind you and for this they thought i was weird. But I don't know if it was really an emotional understanding or an academic one. It's like seeing a movie where a kid gets kidnapped before and after being a father (or so I'm told). There always remains some disconnect till it's your turn or until the concept becomes personal.
I had a friend die, some family, a grandmother too. Some deaths random, some pretty much expected. I don't know that death got anymore real for me at anytime. Maybe we only let it get so real or it would consume us. Worrying about it, would make it harder to live. I don't know if that's for the better or worse. A man cursed to always think about his impending demise would despair, he might throw himself headlong into building a legacy. Which could either make people remember he lived or deprive him of any happiness.