The Chachamim say (Megillah 24a) that a poche’ach, a person whose clothes do not cover his body in a respectable manner, may not read from the Torah nor lead the prayer service. Therefore, a person wearing a sleeveless shirt or shorts may not be appointed chazan.
If his sleeves are very short and do not reach near his elbow, he should cover his arms until his elbow with his tallit. However, one whose short sleeves almost reach his elbow may serve as chazan.
Likewise, the Chachamim state that a person whose beard is not yet full may not be appointed a regular chazan, since it is disrespectful towards the congregation and the prayer service to select someone who has not yet completely matured. However, on a temporary basis, even an adolescent whose beard has not yet started to grow may lead the prayer service.
An eighteen year-old whose beard has started to grow slightly may be appointed a regular chazan. Similarly, appointing a twenty year-old whose beard has not yet sprouted is permissible, since it is clear to all that he is an adult and there is no disgrace to the congregation or to the prayer service (Shulchan Aruch 53:6-8).
Based on this, some Acharonim write (Pri Megadim; Bei’ur Halachah) that a young boy who has lost one of his parents may not be a regular chazan for Shacharit and Minchah, rather he should suffice with saying Kaddish. But for Ma’ariv, in which there is no Chazarat HaShatz, even one whose beard has not yet grown in completely may lead the prayer service regularly.
.Rashi interprets “poche’ach” as one whose thighs are showing. It is from here we learn that one’s legs must be covered at least until the knee. I do not distinguish here between above the knee and below it, because today most people do not usually stand in front of important individuals with knee-length pants, and therefore the chazan is prohibited from wearing such pants. However, in kibbutzim or youth organizations, where the people regularly wear shorts even when they approach respected people, it is permissible for the chazan to wear pants that only reach the knee.
Concerning the upper limbs: The Rif, Rambam, and Tur maintain that a poche’ach is one whose shoulders are exposed. Therefore, if one wears a shirt with very short sleeves yet his shoulders are covered, he may be appointed chazan. That is how the Mishnah Berurah 53:39 rules. However, the Shulchan Aruch 53:13 rules based on the Itur, and so rules the Bach, that even if one’s arms are showing, he is considered a poche’ach. According to this, the chazan must be wearing sleeves that reach his elbow. However, it seems that even according to the Shulchan Aruch it is not necessary to measure exactly, but near the elbow is also considered until the elbow itself. There are those who are stringent that the chazan must wear long sleeves because it is accepted and modest to go before prominent people in that manner (Yaskil Avdi, part 7, p. 329). Nevertheless, since the Rif, Rambam, and most poskim are completely lenient regarding arms, one may be lenient and permit sleeves that reach close to the elbow. Those who seek to glorify the mitzvah cover their entire arm with a long sleeve. My rabbi and teacher, HaRav Tzvi Yehudah HaKohen Kook, would meticulously insist that the chazanim wore long sleeves.
.In Chullin 24b it is written, “Once a person’s beard has filled in he is fit to become an emissary of the community and descend before the ark.” The Tosafot ask: but didn’t we learn in Megillah 24a that even a 13-year-old boy may descend before the ark? They answer that he may be a chazan temporarily, but not on a regular basis, nor for fast days. That is also the opinion of most of the Rishonim. However, the Ramban and the Ran maintain that one whose beard has not yet filled in may not serve as chazan even temporarily, and only if no one else there can lead, a 13-year-old may be appointed. Additionally, although one who shaves is considered someone whose beard has filled in, my rabbi and teacher, Rav Tzvi Yehudah Kook, was accustomed to enhance the mitzvah by only appointing a fully bearded person to be chazan. (Perhaps in doing so he also intended to encourage the yeshiva boys to grow beards of their own).
Concerning a young boy who is mourning the death of one of his parents, the Pri Megadim and Bei’ur Halachah 53:6 s.v. “Yuchal,” write that he may not be a regular chazan. Shut Shivat Tzion 18 writes that where he lived the minhag was not to be strict about this. Instead they let the mourning youths lead the services throughout the whole year, since the congregation relinquishes its honor. However, it should be noted that only according to the Rambam a congregation can relinquish its honor, but according to the Rosh, a congregation does not have the authority to do so (because there is also the matter of the respect of Heaven, kevod Shamayim). Furthermore, the Acharonim discuss the case of a yeshiva of young adults. Beit Baruch 29:45 maintains that because all of them are young, they surely relinquish their honor. Again, his words are justified based solely on the opinion of the Rambam. Moreover, if there are 15 and 16-year-olds present, there are almost always some there whose beards have filled in. Since this is a matter of uncertainty, the rabbi of the yeshiva must determine what is best according to the situation at hand.
The Magen Avraham, brought by the Mishnah Berurah 53:25, writes that it is not necessary to testify whether or not a 13-year-old has two hairs, for we rely on the presumption (chazakah) that he has. However, the Pri Megadim writes that according to the Rambam, who maintains that prayer is a biblical commandment, testimony is necessary. Nevertheless, it seems that since today the chazan does not fulfill a person’s obligation to pray for him, all poskim agree that testimony is unnecessary.