At 5 years old I genuinely believed that I would be an Olympic swimmer (freestyle medalist) because because my swimming coach said so and because I could hold my breath and swim a lap of the (small) pool underwater.
6 year old me was going to be an astronaut of course! I remember sitting cross-legged on the floor with a hundred other primary kids in the main hall at school. Our school had one black-and-white television tilt-propped on top of a 6' tall metal stand with black legs on wheels. We watched and listened in silent awe to the crackly sound and the smudgy image as Neil Armstrong jumped off the lander's steps. The idea of low gravity was so cool.
8 year old me was known as the 'mad scientist' because of explosive chemistry experiments I conducted in the morning before my paper-run, breakfast and school (I still carry the scars).
10 year old me was focusing on the application of his sketchy chemistry knowledge by expanding into rocketry, launching ambitious (mostly failures) multi-stage rockets into the (very low) atmosphere (more scars).