The 12-year-old boy appeared to grow black hair is a normal phenomenon. 12-year-old boys appear long black hair, because into puberty, the appearance of secondary sexual characteristics, manifested in the pubic area, under the armpits have hair growth, there will also be beard situation; in addition to the appearance of hair thicker increase, there will also be a laryngeal knot, the voice appears to change, the height appears to grow and other secondary sexual characteristics of the performance. The density of body hair is affected by the level of hormones in the body, and also has a certain relationship with genetic factors, parents are advised to pay attention to observe the growth of body hair, pay attention to observe the development of the genitals, and observe whether there is spermatorrhea and other problems. 12-year-old children into the second sex characteristics, found that the development of abnormalities, it is recommended to consult a regular hospital specialist guidance.