I rediscovered most of the more widely agreed upon ontological categories (minus one that I still don't believe to adhere to the definition) before I knew they were called that, at about the age of 17. The idea of researching them came to me after reading a question from some stupid personality quiz they gave us in high school, something like "If you were a color, which color would you be?" -- and something about it rubbed me the wrong way, it just felt ontologically wrong, conflating entities with properties like that. (Yes, I did get the intended meaning of the question, I wasn't that much of an Aspie even back then, but I could also see it in the other, more literal way.)

I remember it was in the same afternoon that I also split up the verb "to be" into its constituent meanings, and named them. It seemed related.