What could be the cause of a girl growing pubic hair at age 8 years?

Get her checked: This may be just a benign happening. I would have my child checked for issues that could effect. Particularly ovary and pituitary.

Andrenarche: This is a sign of the output of the adrenal glands rather than the hormones of puberty. It can be seen as early as 6.5 yrs and accompanied by underarm odor. The hair is thin and silky. If there are no other signs of puberty, ie. Breast buds or an accelerated growth rate, this can be discussed when you are next in the docs office.