A minor is not punishable if he transgresses a Torah law. When he matures he is punishable; the Torah calls him a man. The early sages (the Tanaaim) determined the age at which a person matures and become liable to punishment: when he grows two body hairs. For a girl this happens after the age of 12 and for a boy after the age of 13. The students discussed the definition of “two hairs.” One of the scholars, Chalbo, ruled that the hairs have to have follicles. Another scholar, Ada son of Ahavah, ruled that if there are follicles, even if no hairs have sprouted, he is considered mature. One of the scholars, Chanina, asked: If follicles are sufficient to rule a person mature, even without hairs, why did the early sages (the Tanaaim) not explicitly state this, instead using the term “two hairs” and not “two follicles”? Answer: the sages purposely did not state their ruling of maturity as “two follicles” even though they knew that a person with two hairless follicles is still mature, for had they formulated their ruling of maturity “until he has two follicles” the formulation would have been misleading and we would have been able to conclude that if a person sprouts two hairs from the same follicle he is not mature. Therefore they used the formulation “grew two hairs,” from which we can conclude that even if the two hairs sprouted from the same follicle he would be considered mature. The scholars asked: Is there a possibility that two hairs will sprout from the same follicle? Is not G-d exalted by the nature which He created, in which though many hairs sprout on one man’s head, each and every hair has its very own follicle? Were two hairs to sprout from the same follicle, they would harm each other; in each follicle only one hair grows. Answer: On a man’s head no two hairs grow from the same follicle, but on the body two hairs can grow from the same follicle. The sages also disagreed about where these two hairs must appear in order to turn the minor into an adult. According to one sage it is sufficient if one hair grows on the back and the other on the stomach, but according to another sage the hairs have to sprout in the same vicinity.
(Babylonian Talmud, Tractate Niddah 52a-b)