I would stay with my grandparents a lot since my parents were young. My mother was just out of high school and my father was traveling a great deal in the Navy. I once caught my grandfather sketching geometric shapes on the back of a church program, so when we got home I pressed him to do it again. He showed me how to construct a cube in such a way that it looked 3 dimensional. I was thrilled and would redundantly draw other things with angles and perceived dimensions!
Recollecting: It all started when I saw my father quickly sketch a human face on a napkin. A few days later, I took the old Garfield cartoon strip I had read and redrew the characters, but replaced the text with my own. I may have been five years old.