I remember when I was fairly young I noticed how "smooth" pulling out the hair on my knees felt. It didn't go far after that because I couldn't find the hairs that would do that very easily.
Then I remember a time on the school bus in 3rd grade (so that makes me 9), for some reason I grabbed my eyelashes between my fingers and pulled on them -- maybe because of irritation or something. I got none to a few eyelashes each time I did that. After that, I found myself in front of the mirror pulling out eyelashes by hand and with tweezers. A whole bunch of my family noticed, plus my teacher, so I got embarrassed and stopped pulling entirely.
I can't say exactly when, but in the later years I started pulling out the hair on my fingers, mainly, but other body areas, too. Around 17 it was my patchy beard until it started growing more evenly, then just some smaller spots (one cheek, corners of mouth, ...).
Now I'm 22 and my fingers and toes are bald, and some annoying irritation has made me go for my eyebrows, resulting in a few blank patches.
Different ages for different areas, I guess

It's a bothersome life we lead...