I would discuss it with the doctor before anything else. I know many people suspect hormones in meat and dairy products but historically, young girls have been hitting puberty earlier and earlier because of better nutrition (average age at one time was more like 16!). Girls that are overweight may have earlier onset of puberty as well. There are so many other factors that it's hard to pinpoint it to just one thing. In my mind, fine blonde underarm hair is less concerning than dark underarm hair.