When I was 11 and 12, I vividly imagined a science fiction story about a group of precocious children and teenagers fleeing nuclear annihilation on a generation ship that they stole and staffed all on their own. I went so far as to vividly dream up a dozen characters, generate several story lines, and even tell some of it out loud to friends. (To their bemusement.)
I never wrote a word of it down, but if I close my eyes, I can still see that ship.
I think that was my very first ever, if unrealized, creative journey.