The Sages inferred from the verse, "Zachor Et Yom Ha’Shabbat Le’kadesho" ("Remember the day of Shabbat, to make it sacred" – Shemot 20:8) the obligation to verbally declare the sanctity of Shabbat. The word "Zochrehu," the Sages understood, refers to verbal declaration ("Zochrehu Be’feh"). There is thus a Torah obligation to recite Kiddush on Friday night after the onset of Shabbat. The Sages enacted an additional requirement to make a declaration specifically over a cup of wine.
The Torah obligation of Kiddush is fulfilled through the recitation of Arbit on Friday night. In the Amida of the Friday night Arbit service, we recite the Beracha, "Baruch Ata Hashem Mekadesh Ha’Shabbat," which makes mention of the sacred status of Shabbat, and we thus fulfill our obligation by reciting this Beracha. After Arbit, we have the Rabbinic obligation to recite Kiddush over a cup of wine.
Men and woman share the same level of obligation with regard to Kiddush. Therefore, if, for whatever reason, a husband cannot recite Kiddush, his wife may recite it on behalf of him and the family. For example, if the husband is ill or has a sore throat, the wife should recite Kiddush for the family.
A child who is below the age of Bar Misva is obligated in Misva observance only for the purposes of Hinuch – training in Misva observance, which is a Rabbinic requirement. Therefore, he is incapable of reciting Kiddush on behalf of adults, as he does not share the same level of obligation. This applies even if the adults already recited Arbit and thereby fulfilled the Torah obligation, such that they now bear only the Rabbinic obligation to recite Kiddush over wine. Although their obligation is on the level of "Mi’de’rabbanan" (Rabbinic enactment), nevertheless, his obligation is on the lower level of "Tereh Mi’de’rabbanan" (literally, "twice Mi’de’rabbanan"). His Misva to begin with is only "Mi’de’rabbanan," since he is a minor, and, in addition, he already recited Arbit, so he has only the requirement of reciting Kiddush over wine. Since his level of obligation is lower, he cannot recite Kiddush for adults.
An interesting situation arises in the case of a boy who completes his thirteenth year and thus becomes a Bar Misva on Friday night, during the summer months. Many people during the summer months accept Shabbat early on Friday evening, before sundown. If a boy’s thirteenth birthday occurs on Friday night, he is not considered a Bar Misva until dark, even if he accepted Shabbat earlier. Although one may accept Shabbat early, he cannot transform that period of time into nighttime. Therefore, the boy is not considered a Bar Misva despite his having accepted the onset of Shabbat, and he thus cannot serve as the Hazan for Arbit, if the service takes place before sundown.
This applies to Kiddush, as well. If the family sits down for Kiddush before sundown, the child may not recite Kiddush for them, as he is not yet considered a Bar Misva. Moreover, in such a case, the boy must repeat Kiddush at nightfall. When he heard Kiddush before dark, he fulfilled the Misva only on the level of Rabbinic enactment, by force of the obligation of Hinuch. Once nightfall sets in, he becomes obligated on the level of Torah obligation, and he must therefore repeat Kiddush to fulfill this obligation.
Many people are not aware of this Halacha, despite the fact that it occurs quite commonly. Parents of a Bar Misva boy must be cognizant of the date when the boy becomes a Bar Misva, and if it falls on Friday night, and the family accepts Shabbat before sundown, the boy must repeat Kiddush after dark.
If the boy becomes Bar Misva on Friday night and the family accepts Shabbat at sundown (as we do during the winter months), then the boy may recite Kiddush on the family’s behalf on Friday night, assuming that he has achieved the level of physical maturity known as "Sheteh Sa’arot" ("two hairs").
Summary: Both men and woman bear a Torah obligation to recite Kiddush on Friday night. A woman may recite Kiddush for a man, but a child below the age of Bar Misva may not recite Kiddush for an adult. If a boy’s thirteenth birthday falls on Friday night, he is not considered a Bar Misva until nightfall, even if he accepts Shabbat before then. In such a case, he must repeat Kiddush once nightfall sets in, at which point he becomes obligated in Misvot on the level of Torah obligation.
