Gemara

(R. Shmuel bar Nachmani): If one says that Yoshiyahu sinned, he errs - "va'Ya'as ha'Yashar b'Einei Hash-m."

Question: If so, how do we explain "...Asher Shav"? (This implies that he sinned and repented!)

Answer: He overturned every judgment he made from the age of eight until 18.

Suggestion: Perhaps he took back the money from the one to whom he gave it!

Rejection: He did this "b'Chol Me'odo" (from his own money).

Rishonim

Rambam (Hilchos Tefilin 1:13): If a minor wrote a Sefer Torah, Tefilin or Mezuzah, it is Pasul, and we put it in Genizah.

Mordechai (Chulin 571): R. Baruch cites Hilchos Shechitah of R. Eldad, who disqualifies Shechitah of someone above 80 or below 18. This is a mere stringency. The custom is unlike this.

Shiltei ha'Giborim (Chulin 1a:1): Some Ba'alei Hora'ah (those who give Halachic rulings) would not allow youths to slaughter until 18 years, like it says in the Mordechai. Then, their minds are settled. This stringency is based on Hilchos Eretz Yisrael. Perhaps they saw a need to make a fence. Some youths receive a Kabalah before 18, and they are lenient about Shechitah. They required 18, for then one is a Bar Da'as and he will not ruin Shechitah.

Rashi (56b DH Ad): When Yoshiyahu was 18, Chilkiyahu found the Sefer Torah, and Yoshiyahu learned meticulously and understood that perhaps he erred.

Poskim

Shulchan Aruch (YD 1:5): l'Chatchilah we do not give to a minor to slaughter if he has not trained his hands to slaughter, until Bar Mitzvah, i.e. 13 years and one day. Some are stringent not to give a Kabalah (certification) to someone less than 18 years, for then he is a Bar Da'as and knows to be careful.

Yam Shel Shlomo (Chulin 1:1): In my youth, I saw that Ba'alei Hora'ah (those who give Halachic rulings) would not allow youths to slaughter until 18 years, when their minds are settled. This stringency is based on Hilchos Eretz Yisrael. Shabbos 56b supports it. The Tur (CM 1) brings that Yoshiyahu retracted everything he judged below 18 years. Also Shechitah needs a settled mind, especially nowadays that we have many stringencies, it is proper to compare this to judging. Everything depends on the youth. Sometimes a youth is a Ben Torah full of Yir'as Shamayim, and Ba'alei Hora'ah agree to consider him an expert. I discuss the Stam case. If he is crude and foolish, we do not establish him [to be an expert] until the proper age, i.e. 18. We find that earlier Chachamim (Gitin 59a) distinguished among Pa'utos (minors who reached a certain level of maturity), based on their sharpness.

Pischei Teshuvah (11): Once a boy is 13 years and a day, he may slaughter in order to feed to dogs, or with others supervising, even if he not an expert and does not know to train his hands.

Shulchan Aruch (CM 7:3): Some say that it is improper to judge if one is younger than 18 years with two hairs.

Beis Yosef (DH Katan): The first opinion learns from (R.) Shmuel. The latter opinion is from the Yerushalmi. R. Yerucham says that it is primary.

Maharsha (56b DH Kol): The Tur learned from here that a judge must be at least 18 years old. What is the source? The verse does not mention 18, only that he became king at the age of eight. The Gemara does not say until Ben Shemoneh Esre (like it said 'from Ben Shemoneh'), rather, until 18. It seems that it refers to until the 18th year of his reign, when they found the Sefer Torah. He was stringent to retract all his rulings.

Levush: A minor cannot judge because he is exempt from Mitzvos. If a 13 year-old is sharp and fluent in Torah law, he can judge with others, or alone, like an expert. Some forbid until 18 years even if he brought two hairs, for then he understands keenly the give and take between people. We find that Yoshiyahu retracted every judgment he made from when he became king until 18, for he did not consider them to be judgments.

SMA (8): After 18 he is a proper man, and he has the strength to save the poor from one who steals from him. The Tur lists this among Midos that a judge must have.

Urim (7) and Tumim (3): This is only l'Chatchilah. Surely, if someone less than 18 judged, the verdict is proper. If not, there is a(nother) case of someone Kosher for testimony and Pasul for judgment. Why didn't Chachamim initially protest against Yoshiyahu for judging below 18? Presumably, he would have heeded them! Rather, since b'Di'eved the verdict is proper, they did not protest against the king. Shlomo started to judge when he was only 12 years old. Since [normally] it is only l'Chatchilah not to do so, and he had Divine Chachmah, it was l'Chatchilah to do so. How could he and Yoshiyahu judge before Bar Mitzvah? All disqualify such judgments even b'Di'eved! Tosfos (Sanhedrin 69b DH b'Yadu'a) says that earlier generations had children earlier, and became adults at the age of eight, like we find regarding Achitofel. (He was only 26 when his great grandson Shlomo was born, so they averaged less than nine years between generations.) Achaz was 11 years older than his son Chizkiyahu.

Shulchan Aruch (ibid.): Some say that from 13 years and above is Kosher, even without two hairs.

Beis Yosef (DH u'Mah): The Tur brings from the Yerushalmi that it is proper to judge from 13 years and above. R. Yerucham says that this is primary.

Birkei Yosef (10): When the Shulchan Aruch brings 'some say... and some say...', he rules like the latter opinion.

Gra (Likut DH v'Yesh): The Yerushalmi says that someone below 20 years without two hairs is Kosher for monetary cases and Pasul for capital cases. This opinion explains Yoshiyahu like Rashi. (He did not retract because he was Pasul for judgment. Rather, he was concerned for lest he had erred.)

Noda bi'Yehudah (2 YD 1): If someone's hands shake, even if he says that he is sure that this does not cause him to pause or press, l'Chatchilah we do not give to him to slaughter. However, b'Di'eved we do not disqualify his Shechitah. Lechem ha'Panim cited Semag in the name of Mahara Stein and the Mordechai to disqualify an old person or one whose hands shake due to weakness or his nature, even if they are sure that they slaughtered properly. Surely this is wrong. Semag preceded them! Also, the Mordechai says that it is a stringency not to allow someone above 80 or below 18 to slaughter. (He did not call it Pigul, i.e. forbidden b'Di'eved, like he said about Shechitah without a Berachah.) Also, the Rema (1:3) says that if one often grows faint, and we know that he is not Muchzak, he is believed to say that he did not grow faint, since he knows Hilchos Shechitah.

Taz (16): The Mordechai brought this from R. Aldad, but said that the Halachah does not follow him. Darchei Moshe says that some are concerned for this. The Maharshal says that it depends on his sharpness and knowledge. I saw people who give Kabalos from the age of 13 if he is expert, has skilled hands and the strength of a man.

Pischei Teshuvah (12): Be'er Heitev brings that we do not let someone above 80 slaughter. Me'ir Nesivim (76) says that nowadays people are weaker, so one should not allow someone above 70 to slaughter, even if he still feels [strong], to avoid distinguishing. Even before 70, Rabanan must supervise, and periodically check thoroughly, elderly Shochtim. They are prone to worsen without realizing that their hands shake.

Teshuvah me'Ahavah (1:118): The questioner wanted to support Hilchos Eretz Yisrael, who disqualifies someone above 80 for Shechitah, from Barzilai ha'Gil'adi, who said 'I am 80. I do not taste the food I eat', i.e. old people have a weak sense of feeling. Shabbos 152a rejected this, for a 92 year old woman in Rebbi's house used to taste the food. (Barzilai lost his sense due to his lust,) This shows that even someone 92 has a keen sense of feel! We say that Kohanim may serve from 13 until they grow old, i.e. until their hands and legs shake due to weakness (Chulin 24b, Rashi). No Shi'ur of years was given. One may slaughter if and only if he has strength and a good sense of feeling. It does not depend on age.