DD2 is 7 months old and is pulling her hair out. She grabs it in her fists and yanks it out. I distract her and remove her hands whenever I catch her doing it (6/7 times a day at least) but she's been doing it for over a month now and it's not making any difference. She's balding on both sides of her head from it, but it hasn't put her off and she's now moving onto the hair at the top of the head.
DD1 didn't have anywhere near enough hair to do this until she was much much older, so I have no idea if it's a developmental thing that will pass or what to do to curb it if it isn't!