From the age of 15-ish, I had a conviction/belief that I would never reach 40.
At the age of 20-ish this gave me an urgency to grab life by the dangly-floppy bits and make sure I experienced every joy available.
To my amazement, I did reach 40, and on the day I did, I:
a) realised that I'd crammed more into my life than my contemporaries
b) was on vacation on a tropical island
c) with a 26 year old pin-up girlfriend
d) got into an awesome fist-fight with a guy who hit on the girlfriend.
I now have 7 years of life that I never expected - all full of wonderful.
My parents are both beyond 70 and have active, hoyful lives.
My S.O.'s parents are approaching 90 and active in hobbies, sports and church.
That said, I'd rather die at 50 with awesome experience under my belt (my current plan) than vegetate to 60, 70, 80 or beyond.
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"I started out with nothin' and I still got most of it left" - Seasick Steve