Posting on behalf of my DSis here...
My DNiece is four and a half. Her hair covers her head, just, but that is it. It has no length at all and is very fine, although it has thickened to some degree in the last year.
Could this be indicative of a vitamin or mineral deficiency? Or some other health issue, do you think? DSis is loathe to bother the Dr with something seemingly so trivial, but it really bothers DN and i guess the unkind comments are likely to increase when she goes to school this September.