I started tutoring math to mostly middle and high school students as a hobby in 1995. I now teach and tutor students from kindergarten to graduate students in every subject from arithmetic to calculus and number theory. My youngest student was 2 1/2 and my oldest was 66.
The best things I've learned have come from Don Cohen's "Calculus By and For Young People - Worksheets" book. The rest I have learned taking independent study classes or just buying a textbook and teaching myself. This last year alone, I (had to) learned differential equations, elementary real analysis, modern algebra and number theory. This would not have been possible without Don's book. Sooner or later I will finish my bachelor's.