I drew loads when I was a kid, and I must have been about 10 or 11 when I realized, had a moment of epiphany really, some of the stuff in the "How you probably draw / how you should draw" section.

I was looking at the cover of the Toy Story VHS and trying to copy it. I remember specifically I was drawing Woody's face, his right cheek to be exact (must have been this photo), and I stopped myself when I realized that what I was about to do was a result of me drawing not what I really saw, but what I thought I should see given my mental model of a face. I think I was about to draw his right cheek too wide or something like that.

That was the first time I ever truly and deeply grasped that concept. I take a certain joy in knowing that I learned it independently.