Niddah, Chapter 5, Mishnah 6. This chapter has been tracing the stages of a person's life and the halachic significance attached to each age. Having established the age from which a girl's relations are a legally recognized act, the Mishnah now turns to the boy and sets his landmark age: nine years and one day. From then on, his act of relations counts halachically, and the Mishnah spells out what follows from that.
"Ben teisha shanim v'yom echad shebba al yevimto, kenanah." Picture a boy who has completed his ninth year and is now one day past it, and who stands bound by the mitzvah of yibum. Should he live with his yevamah, the yibum has been carried out and she is his wife. That is striking, since a minor normally has no ability to perform a kinyan, an act of legal acquisition; such power arrives only at thirteen, with bar mitzvah. Here, however, his act does take hold, thanks to a dispensation that the Torah granted for yibum alone.
Precisely because this is a unique allowance limited to yibum, it does not carry over to the rest of his legal powers. "V'eino nosein get ad sheyagdil." If he later decides that he wishes to end the marriage, he cannot validly give her a get until he grows up and reaches thirteen. The Torah's special rule gave him the ability to acquire through yibum; it did not grant him the general standing of an adult who can execute a divorce.
"U'mitamei mishkav tachton k'elyon." Once he has reached this age, relations with a woman who is a niddah give him the status of a boel niddah. The tumah he transmits downward is then unusually potent: bedding at the very bottom of a pile beneath him becomes tamei to the identical degree as the layer on top.
"U'posel v'eino ma'achil b'terumah." There are two halves to this clause. On the one hand, he can disqualify: his relations with a woman are capable of rendering her ineligible to eat terumah. On the other hand, he cannot yet confer that eligibility. If he himself is a kohen and he marries, he does not have the power to bring his wife into the household so that she may eat terumah. That begins only at thirteen, once again because his kinyan does not carry the full standing of an adult's acquisition until then.
"U'posel es habehemah mei'al gabei hamizbeach, v'niskeles al yado." If he has relations with an animal, the act counts as bestiality in halachah. As a result, he renders that animal unfit to be brought as a korban on the mizbe'ach, and the animal is liable to be stoned on his account.
"U'ba al achas mikol ha'arayos ha'amuros baTorah, mumasin al yado v'hu patur." If he has relations with any of the forbidden relatives listed in the Torah, the women are put to death on his account, since a real prohibited act has taken place. He himself, however, is exempt, because he has not yet reached the age at which a person carries legal responsibility for his actions.
The Mishnah thus sets out a single dividing line for the boy at nine years and one day. From that age his relations are a halachically real act, with consequences in yibum, in the tumah of a boel niddah, in terumah, in the fitness of an animal for the mizbe'ach and in the arayos, while the fuller powers of an adult, giving a get and conferring terumah upon his wife, wait until he reaches thirteen.