Niddah 5:4-5

Niddah 5:4

A girl three years and one day old can be betrothed through relations and if a yavam is intimate with her, he has performed an effective yibum. [Note: This is not discussing the advisability of such an act, merely the efficacy.] One can be liable for adultery because of her and she can convey ritual impurity to one who is intimate with her so that the lower mattress on which he sits is ritually impure like the upper mattress. If married to a kohein, she may eat trumah; if an unfit person is intimate with her, he invalidates her from marrying a kohein. If a forbidden relationship is intimate with her, he is executed but she is not. Younger than this age, relations are like putting a finger in the eye (i.e., not halachically effective).

Niddah 5:5

If a boy nine years and one day old is intimate with his yevama, he performs an effective yibum but he is unable to divorce her until he reaches the age of majority. He can be rendered unclean by a niddah so that the lower mattress on which he sits is ritually impure like the upper mattress. He can disqualify (the daughter of a kohein) from eating trumah but he cannot qualify a woman to eat trumah. He renders an animal unfit as a sacrifice (through relations with it) and causes it to be executed by stoning. If he is intimate with a forbidden relationship, she is executed but he is not.