Well, it could have been when I was sitting in front of the TV and watched the first moon landing as a kid, or it could have been when I got my first globe when I was around two, I can't be sure, I was pretty young back then. There has never been a time when I did not think the earth was round, that I am aware of.
Then a wonderful thing happened, I got to fly on the SST from France to New York and we flew high enough that I could actually see the curvature of the earth. It inspired me enough to go and get my pilot's license, and I've been flying for over 20 years now. Also, I assure you that gravity is real, just ask, IF YOU COULD, my father-in-law who fell from the sky in his 1945 Corsair in November 2002, and died.