Can I answer? I'm only 35, but I like this question!
I can't remember the exact date I became aware of my own mortality, but it was pretty young, in my early twenties somewhere. I just suddenly 'got it', that I was going to die. And it was fine. I spent a lot of time thinking about it, and acknowledging that it's a very sad thing that we don't get to hang around this Earth forever. I will always feel sad about dying, my own death and the deaths of other people, but I've accepted that it's okay to be sad about it. In the rest of my life that knowledge that I'm going to die is kind of with me all the time, but not in a morbid and melancholy way, in a 'I'M STILL HERE, HOW FANTASTIC!' kind of way. So that every day is a gift, and the fact that I'm here at all in the first place is pretty bloody brilliant.
I think it depends on whether you dwell on the fact that you're going to die, or perhaps just treasure the fact that you're still here. Both of my grandmothers are still around, and each has the opposite attitude of the other: my Grandma Dora has been telling us for years that she's not got long left, but she's still going strong; my Grandma Freda has never really mentioned dying and has always just seemed happy to be alive, and has always made the most of every day. Out of the two I would say that my Grandma Freda is the happier of the two in her old age.
My friend lost her husband this week (he was only 37), and it doesn't make sense to me that we should only start thinking about death when we think we're approaching it. I think we should talk about it more from a young age, so that we can accept it and learn not to be scared of it.