May a child above Bar or Bas Mitzvah recite Kiddush on behalf of an adult:[1]

Children above the age of Bar or Bas Mitzvah are invalid to perform [the night[2]] Kiddush on behalf of adults, whether male or female, until one knows for certain that the child has developed the Halachically valid signs of puberty.[3] Thus, a [girl or even] a boy who is above the age of 13 cannot be Motzi even a woman [i.e. his or her mother] for Kiddush, if one does not know that he [or she] has grown the puberty signs.[4] Therefore, women are to make Kiddush themselves [rather than to hear from this questionable child]. If the woman [or man] does not know how to say the Kiddush, then she [or he] is to repeat word after word, after the child.[5] [The above, however, only applies to the Friday night Kiddush which is a Biblical obligation. However, by the Shabbos day Kiddush, and the Kiddush of Yom Tov by night and day, which is only a Rabbinical obligation[6], a child above Bar/Bas Mitzvah may be Motzi other adults even if the signs of puberty have not yet been ascertained.[7] Likewise, even on Friday night, if everyone being Yotzei has already Davened Maariv, including the woman, then this child above Bar/Bas Mitzvah may be Motzi them.[8] If, however, one knows for certain that the child has yet to grow the signs of puberty, then they cannot be Motzi other adults even by the daytime Kiddush.[9]]

The puberty signs: The following are the valid puberty signs to determine that a Bar or Bas Mitzvah child has reached Biblical adulthood, is Biblically obligated in all Mitzvos, and can be Motzi others:

1. Two pubic hairs:[10] If the Bar or Bas Mitzvah child has grown two pubic hairs they are considered a full-fledged adult. Prior to growing two pubic hairs the child is not considered Biblically obligated in the commands. [The child himself is not trusted to attest for this, and it is only valid if seen by the adult or testified by other adults.[11] Owing to the inability to physically ascertain this, due to obvious reasons of privacy and modesty, the other more apparent signs which attest to the growth of pubic hair must be used. If these signs are absent, then one must treat the child as belonging to the questionable category, and not be Yotzei with him any Biblical command, including the night Kiddush.]
2. Beard:[12] Once a male child grows an abundance of facial hair, he is assumed to have grown two pubic hairs even if one has not ascertained this to be of fact.
3. Eighteen years old:[13] Once a child has reached the age of 18, they are assumed to have grown the two pubic hairs even if one has not ascertained this to be of fact [unless one knows for certain that he or she hasn’t[14]], and may be Motzi others even for a Biblical command.
4. Saris-36 years old:[15] Once a person reaches 36 years of age, or has developed the signs of a Saris, they are considered Biblically obligated in all commands even if they have never developed signs of puberty.

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[1] Admur 271:7; M”A 271:2; M”B 271:3; Rav Poalim 1:10; Kaf Hachaim 271:9

[2] See later on regarding the day Kiddush

[3] The reason: As a child above Bar/Bas Mitzvah only becomes Biblically obligated in the commands upon reaching puberty and growing two pubic hairs. If one does not know this to be the case, he is only obligated due to doubt, as perhaps he has not yet developed, and is hence Biblically exempt from all the commands. Accordingly, he cannot be Motzi an adult, as one of questionable obligation cannot be Motzi one of certain Biblical obligation. [Admur ibid; 37:3; 39:1 regarding Tefillin; 55:6; 128:49; 199:9 regarding Birchas Hamazon; M”A ibid; M”B ibid]

[4] The reason: As a woman is Biblically obligated in [the nighttime] Kiddush without a doubt, and this child is only obligated due to doubt, as perhaps he has not yet developed two hairs, and is hence Biblically exempt from all the commands. [Admur ibid]

[5] Admur ibid; M”A 193:2

[6] Admur 271:4 regarding Yom Tov; 289:2 regarding daytime; M”B 271:2

[7] See Admur 55:6 regarding Minyan; 199:9 regarding Zimun; Rama 55:5; Maharik 49; All Poskim in next footnote regarding Friday night after Maariv

The reason: As the daytime Kiddush, and Kiddush of Yom Tov, is merely a Rabbinical obligation, and by a Rabbinical obligation, we rely on the Chazaka of Rava that a child above Bar/Bas Mitzvah has grown two pubic hairs, even if this has yet to be verified. [See Poskim ibid]

[8] Elya Raba 271:3; Degul Merivava 271; Derech Hachaim; M”B 271:2-3 in end; Biur Halacha 271:1 “Miyad” in end; Shaar Hatziyon 271:2; Kaf Hachaim 271:9

Background-If one already Davened Maariv: Even on Friday night, if everyone being Yotzei has already Davened Maariv, including the woman, then technically this child above Bar/Bas Mitzvah may be Motzi them. [Poskim ibid] This applies whether or not the child himself has already Davened Maariv, as once one has mentioned Shabbos in Maariv Shemoneh Esrei he is only Rabbinically obligated in the Mitzvah of Kiddush [See Admur 271:2; M”A 271:1; Rameh Mipuno 2], and we thus apply the same law as by all Rabbinical commands, that the chiodl ha a Chazaka to have brought two hairs. [See Poskim ibid] Now, although there is room to suspect that perhaps the people who Davened Maariv did not have in mind to be Yotzi the Biblical Mitzvah of Kiddush in doing so, as well as other suspicions, nevertheless, this is a mere doubt which hence creates a Sfek Sfeika, as perhaps the child has grown pubic hairs, and hence he may be Motzi other with Kiddush. [Biur Halacha ibid; Shaar Hatziyon 271:7]

[9] The reason: As Trei Derabanon [i.e. 1) child 2) Rabbinical Kiddush] cannot be Motzi Chad Derabanon. [Derech Chaim ibid; Shaar Hatizyon ibid; So rule regarding Megillah: Beis Yosef 689 in name of Levush; Olas Shabbos 689:3; Peri Megadim 689 A”A 1; M”B 689:6; Kaf Hachaim 689:11; So rule regarding Chanukah: Peri Chadash 675; Olas Shmuel 105; Ben Ish Chaiy Vayeishev 19; M”B 675:13; Kaf Hachaim 675:26]

Friday night: According to the above, if the child has not yet Davened Maariv, while the adult has Davened Maariv, then he can be Motzi the adult even if he has not yet grown two hairs, and even if he is below the age of Bar/Bas Mitzvah, as they are both now Chad Derabanon. [Degul Merivava 271; Rav Akiva Eiger Teshuvah 7; Derech Hachaim ibid; M”B 271:2; Shaar Hatziyon 271:2 and 4; Kaf Hachaim 271:9] However, if the child already Davened Maariv, then he cannot be Motzi other adults even if they had Davened Maariv, as this reverts back to a case of Trei Derabanon. [Degul Merivav ibid; Derech Chaim ibid; Shaar Hatziyon ibid] However, some Poskim rule that the child can be Motzi the adult even in such a case, as being that the child could potentially be a Chad Derabanon, if he were not to have Davened Maariv, then he can be Motzi another Chad Derabanon, even though he is currently a Trei Derabanon. [Rav Akiva Eiger ibid; Chayeh Adam 5:23, brought in Shaar Hatziyon 271:2; See Kaf Hachaim 271:9] On the other hand, some Poskim rule that one can never rely on the fact that the adults have Davened Maariv, as perhaps they did not have in mind to be Yotzei the Biblical Mitzvah of Kiddush in doing so, and hence retain a Biblical obligation. Accordingly, even if the child has not Davened Maariv he cannot be Motzi an adult who Davened Maariv. [Tosefes Shabbos, brought in Shaar Hatziyon 271:4; M”B 271:2; Conclusion of Kaf Hachaim 271:9]

[10] Admur 271:7; 37:3; 39:1 regarding Tefillin; 55:6

[11] See Admur 271:7 “Does not know for certain”; 39:1 “If he does not know for certain that he brought two hairs”

[12] M”A 271:2; Admur 199:9; Michaber C.M. 35; M”B 271:3; Shaar Hatziyon 271:5

[13] Admur 39:1; M”A 39:1; See Miasef Lekol Hamachanos 39:7; Chikrei Halachos 5:62; Yagdil Torah N.Y. 8:44

[14] Admur 55:6

[15] Admur 55:6; Michaber 55:5; Nida 47b;

[16] See Rav SZ”a in SHH”K 51 footnote 38; Piskeiy Teshuvos 271:7

[17] The reason: As to everyone else he is a Safek, and perhaps one of the other children has already grown the signs and is a definite obligation, and thus we revert back to the same law ruled by Admur ibid. [Rav SZ”A ibid]

[18] Otherwise, the child knows for sure he is only Rabbinically obligated and certainly he can be Yotzei with another child.