Small dark hairs on the chin of a young female. Why?

Depends on age. It is normal for girls to have fine (vellus) hair on the face. At puberty, some girls will develop a few dark (terminal) hairs as circulating hormones (estrogen, testosterone) increase. Heredity is a strong predictor of facial hair and it is common for women in certain ethnic groups. Heavy facial hair (hirsutism) is abnormal especially if accompanied by voice change, irregular periods and balding.