When did you first become aware of your parents' mortality?

My parents are in their 60s and I'm well-aware that they're not going to be around forever, but it was really hammered into me last night when my dad had a mild heart attack. It's difficult for me to even imagine life without my parents since they've [i]always been there[/i] for my 28 years of existence. But all of my grandparents are deceased so I know the day will come when I have to say goodbye to my mom or dad.