I think I was maybe 8-ish when I realized everything I experienced was actually a memory of a past event, and that memory is incomplete and unreliable, and so what I believed to be real was really only some nature of a guess based on things I may or may not be recalling correctly, and always incompletely. And that everyone else is in the same boat. But I was impressionable, so when mom told me that was stupid I discarded the thought for another 30 or so years.
As a little kid I was very interested in astronomy (still am). I remember telling my parents all the stuff I’d learned from reading – the distance from the earth to the sun, all the planets in the solar system, etc.
I still think it’s remarkable that at some very young age, with only the money in my piggy bank and mostly stuff I had lying around the house, I built a pretty decent small reflector telescope which actually worked rather well. The only things I bought were the primary and secondary mirrors. Everything else was homemade or improvised. I was pretty proud of myself the first time I saw the rings of Saturn!
There’s a bigger word/meaning than “desire to live” I am reaching for, but I am not coming up with anything. It is not consciousness. I’m no philosopher. It isn’t about healing per se, it’s about continually knitting yourself up into a being – healing wounds is simply a gross manifestation of this. Breathing, photosynthesizing, are other manifestations. I’m just not able to describe this very well, sorry.
I was in the 4th grade, so maybe 10 years old (?) when I worked out that if you add up successive powers of 2 you get 2 less than the next power of 2. For example, 2+4+8+16 = 30, which is 2 less than 32. I didn’t have a proof, but I noticed the pattern; not something most elementary school students would do.
I was also really interested in astronomy. Even elementary school I knew a lot about the solar system (this was around the time of the Voyager missions) like the fact that it takes Saturn 29 years to go once around the sun.
Dinosaurs, of course. A very common obsession with the younger set, reinforced in my case with frequent trips to the American Museum of Natural History to worship at their shrine. I could at one time have given a pretty reasonable rundown on then current dinosaur taxonomy (which is no longer even remotely current).
Sadly I did not retain that obsession long and the vast majority of my knowledge base there is gone, like tears in the rain.
However my junior high fascination with the Visigoths (they were the genesis) and all things historical stuck.
According to my parents, I made a similar announcement around that age.
I also wondered if reality was an elaborate charade or illusion being staged for my benefit, and only the places I regularly visited (my neighborhood, my school, the grocery store) were permanent. Faraway places like London and Egypt didn’t actually exist, but if you traveled to one of those locations, “they” would hurriedly build enough of it to fool you. I didn’t tell my parents about this one, but they undoubtedly would have found it hilarious.
As a teenager, I figured out on my own that if one is unsure whether God exists, it’s a smart bet to do what He wants us to, just in case. Much later I learned that this is called Pascal’s wager.
And it’s a terrible argument. Especially because it ignores the possibility of all the other possible gods and what they might want.
I was 7yo, and in Sunday School we were learning about monotheism. The implication was that believing in one God was somehow “better” or “more advanced” than believing in many Gods. I raised my hand and asked the teacher, “What if, some day, we discover that there really are many Gods? What if we discover that there are no Gods?” The teacher answered, “Well, we believe that there is only one God.” She never did answer my question.
Me too. My old man was a fighter pilot, an engineer and a bunch of other fancy stuff. He was big into RC planes, from the early, early days of the hobby. He would drag me to the shop and we would build airplanes, and they drag me to Hell on Earth (bum-fuck drylake in the middle of Nowhere) to fly them. So, I learned more than most kids my age.
And now… the rest of the story…
My two best friends are the ones who became pilots. I hate to fly, never wanted to learn. And that fuckin’ drylake I hated so much? I live there now.
I was about that age, too. My younger sister always laments that it took her so long to figure it out.
I was absolutely terrified of the atomic bomb starting when I was about three. It colored a lot of my childhood and my nightmares. I still have an extremely clear memory of my mom watching TV one morning while she was ironing. I remember all the details of the living room as it was then, the TV cabinet my dad built for our 11" TV, the cafe curtains with the bobbles on them. I remember exactly what I was wearing. It’s my only memory of my mom before her hair started to grey (relatively) prematurely. I can remember exactly what picture was on TV (I much later realized it was the UN security council), and I asked my mom what the men were talking about. She said they were talking about whether we were going to have a war.
I walked outside, and the rest of the day I kept thinking the bomb was going to fall on us any minute. It was terrifying.
I was similarly obsessed with cars at that age, and thanks to that I had a reasonably good understand of how an internal combustion engine worked. I’m sure there were a lot of details I was missing, but I knew that gasoline went into the cylinders, where it got ignited by the spark plug, which made the piston move, which made the crankshaft rotate.
I figured out that if there is an all-knowing God, he/she isn’t going to be that easy to fool.
It wasn’t something that I spontaneously figured out like a lot of the replies here, but my dad explained alternate bases (binary, hexadecimal, etc.) to me many many years before it ever came up in school, and it completely clicked for me. The conversation started with me asking why his calculator had “December and October buttons.” 
My dad had a lot of skill with mathematics and electronics, and his eagerness to share his knowledge with his four daughters before we were “old enough” is something that really helped me throughout life.
Because 31 Oct = 25 Dec ? (Halloween = Christmas)
My Big Brother, 12 years older than me, was majoring in Math in college while I was in 4th or 5th grade. He was into astronomy, and had a telescope. He let me look at stars through it.
As everyone knows (right?), when you look at a star through a telescope, it appears to move visibly faster than when you look at the same star with the naked eyes (in which case it doesn’t visibly move at all). He asked me if I could figure out why that was so.
I immediately came up with the right answer.
There are some geometric puzzles where you are shown some kind of geometric figure made of line segments and vertices, and you are asked to draw the figure without lifting your pencil from the paper or retracing lines. Big Brother asked me to figure out what property the diagram must have in order for that to be possible. I figured that out in about 15 seconds.
I think most of us who grew up in the Cold War had similar moments. I did.
Mom had a similar attitude towards power tools. I think my sister and I first used the band saw (heavily supervised, of course!) when we were about 5.
I discovered that there was no Santa by a more mundane method. My mother took me to pick out presents, and I said there was no Santa Claus.
To the OP, having watched my father repair TV sets, when of pre school age, I impressed my grandmother by fine tuning an analog TV set.
nods
My dad was a nuclear physicist for Los Alamos and a physics professor at the local college. At one point my mom showed me a map of the world and pointed out the US and across the water the very very large USSR. Said they had more bombs than we did and that their entire purpose in life was to take over everyone and put them under the thumb of an authoritarian dictatorship. And we were the only real opposition to that, and the only thing stopping them from bombing us was fear of immediate retaliation, but at any moment someone on either side could panic or blink and decide they had to go first.
Yeah, thanks for the nightmares.
Me too. However, I also reasoned that presents were a good thing, so cynic that I was, I kept my mouth shut.