Aye, what Barny said.
I laid in bed the other night and had a think about life... I'm 21 years old. If I lived till 80, that's 1/4 of my life gone already and that seems scary. But on the other hand, I've been alive for 183960 hours. In that time I could've watched 367920 boxing bouts (if the average time for each was 30 minutes).
It's hard not worry when you simplify your life into fractions and we'll never comprehend our own end. But you should use this as a wake-up call to experience everything you possibly can, do things you wanted to but never have. I feel like humans just fall into a groove, become robots and time passes them by.
I think as well that old people (even older than you Cua
) shorten their lives when they reach a certain age and just 'wait' for the end. Why aren't there more older people exercising? That 93 year old bodybuilder is a perfect example, it's like they give up... I understand some have medical problems but I will ensure that I exercise regularly at the age of 60...70...until my legs crumble to dust and my joints seize up, I'll make sure I keep my heart exercised.
So in conclusion, you may have lived half your life or less already. But think of what you've done and what you could do, don't look at in a fraction, look at in hours and think about what can be done in an hour. Optimism keeps you alive longer too.