I started somewhere between 4-5 years old. Ever since I was in kindergarten I drew on pretty much every paper I was given. Tests, homework, essays, didn't matter to me. Eventually (middle school) I started keeping sketchbooks that I drew more complete pieces in. Once I got into highschool though I started learning how to properly draw (sketches, lineart, oh my!) and I've improved more over 1 year than I have since before highschool. I'm actually proud of some of my pieces now! I don't know what on Earth motivated me to start drawing, or to continue for that matter, but I'm glad I did get into it.
Comparison between 2-3 years ago and now.
http://imgur.com/a/VFKaS