1. The Gemara says that a girl can become pregnant only after the age of twelve.
2. A girl who engages in forbidden relations under the age of three may still marry a Kohen.
3. The Mishnah lists laws that apply to a nine-year-old boy.
4. The Mishnah discusses the laws of a girl who makes a vow.
5. The Mishnah similarly discusses the laws of a boy who makes a vow.
A BIT MORE
1. After citing an incident wherein a Nochri woman claimed to have had a child at age seven, the Gemara explains that either a Nochris can become pregnant earlier or she was lying.
2. There is a law that a Kohen may not marry a Zonah, which includes any woman who had relations with someone forbidden to her by the Torah. If she has the relations when she is under the age of three, this does not constitute an act of relations and she may still marry a Kohen.
3. For example, if he has relations with his Yevamah, he becomes married to her. If he has relations with a woman who is forbidden to him, she will be punished by Beis Din while he is exempt (because he is a minor).
4. Before the age of eleven, her vows are not considered valid. After the age of eleven, her vows are valid only if we ascertain that she understands to Whom she vowed. Once she is over twelve, her vows are assumed to be valid, but we check her vows throughout her twelfth year (after which her vows are always considered vows and checking her level of intellectual maturity is unnecessary).
5. The laws are the same as above (#4), but the ages of eleven and twelve are replaced with the ages of twelve and thirteen respectively. This (#4 and #5) is the opinion of Rebbi. Rebbi Shimon ben Elazar maintains that a boy's vows are checked at eleven while a girl's are checked at twelve (the opposite of Rebbi's opinion).