Yevamos, Chapter Twelve, Mishnah 4. The Mishnah deals with the validity of chalitzah when one of the parties lacks full mental capacity or has not reached the age of mitzvos.
"Cheresh shenechlatz, v'cheresh shechaltzo, v'chalatz l'katan - chalitzatah pesulah":
"Cheresh shenechlatz" - a deaf-mute who receives the chalitzah, that is, chalitzah is performed upon him.
"V'cheresh shechaltzo" - the yevamah herself is a deaf-mute, and she is the one performing the chalitzah.
"V'chalatz l'katan" - the yavam is a minor who has not reached the age of bar mitzvah, and chalitzah is performed upon him.
In all of these cases the chalitzah is invalid: a deaf-mute can neither perform chalitzah nor have chalitzah performed upon him, and a minor cannot be the one upon whom chalitzah is performed.
"Ketanah shechaltzo tachlotz mishetagdil":
A minor girl who has not reached the age of bas mitzvah and performed chalitzah must perform chalitzah again once she grows up, so that the chalitzah will be valid. "V'im lo chaltzo - chalitzatah pesulah" - if she did not perform chalitzah again after reaching the age of bas mitzvah, the chalitzah she performed while a minor is invalid. Without a new chalitzah, the earlier one does not suffice to release her and permit her to marry any other Jew besides the yavam.
A variant text:
Some have a different version of this Mishnah regarding the minor girl: "Ketanah shechaltzah - chalitzatah kesheirah" - meaning that the chalitzah she performed while a minor is valid. This is the second version of this Mishnah.
In summary: We learned that the chalitzah of a deaf-mute, whether as the one performing it or as the one upon whom it is performed, as well as chalitzah performed upon a minor yavam, are invalid. A minor girl who performed chalitzah must perform it again once she grows up, and if she did not do so, her first chalitzah is invalid and does not permit her to marry anyone else, while according to another version her chalitzah is valid.