Gemara

46a (Mishnah) Question: How may Reuven separate Chalah from a Tamei dough on Yom Tov?

Answer #1 (R. Eliezer): He does not specify which part is Chalah until it is baked.

Answer #2 (Ben Beseira): He [separates Chalah, and] puts it into cold water to impede Chimutz.

66b (R. Yochanan): "Ish Ish Ki Yihyeh Tamei la'Nefesh" - an Ish (individual) is detained to Pesach Sheni, but a Tzibur is not. Rather, the Tzibur brings it b'Tum'ah.

67a (Abaye): It could have said just "Ish Ish Ki Yihyeh Tamei." "La'Nefesh" is extra [to exclude Zav and Metzora. They are more stringent than Tum'as Mes. They are not permitted b'Tzibur.]

Bechoros 27a (Shmuel): A Tamei Kohen may eat Terumas Chutz la'Aretz, unless the Tum'ah comes from the Kohen's own body (e.g. a Zav or Nidah);

(Ravina): Therefore, a Nidah may separate Chalah for a young Kohen to eat.

Nidah 32b (Beraisa - R. Yehudah): "Ish Ish" includes even a one day old boy. (Three emissions of Zivah make him a Zav);

R. Yishmael, son of R. Yochanan ben Brokah learns from "la'Zachar ul'Nekevah", i.e. a male or female of any age.

Observation: When the Torah includes (minor) boys, it includes even a one-day-old.

Contradiction (Beraisa) Suggestion: Perhaps "Ish (who has Keri is Tamei)" applies only to an adult!

Rejection: "V'Ish" includes even a nine-year-old boy.

Answer (Rava): One of these (Keri) is a tradition from Moshe from Sinai. The verse is only an Asmachta [regarding Zivah].

Question (Reish Lakish): If a boy saw one sighting of Zov, does the Zov have Tum'as Maga (of touching)?

Perhaps "Zos Toras ha'Zav va'Asher Tetzei Mimenu Shichvas Zera" teaches that a man's first sighting of Zov is Tamei only if his semen is Tamei;

Or, since a second sighting would join to make him a Zav, even the first sighting has Tum'as Maga!

Answer (Rava): "V'Zos Toras ha'Zav" applies whether he is small or an adult. Just like an adult's first sighting of Zov is Tamei, also a child's.

Rishonim

Rif (14b): The Mishnah applies in Eretz Yisrael, where Chalah is mid'Oraisa. No Kohen may eat Tamei Chalah there at all. Chalas Chutz la'Aretz is mid'Rabanan. A minor may eat it. Regarding a man, this is until he sees Keri. For a woman, it is until she becomes Nidah. If there is no minor Kohen we do like R. Eliezer.

Ran (DH v'Chasav): Others disagreed. When there is no minor around, we may bake for an adult. He will immerse! This is wrong. Now he is not ready. We do not say "he could immerse", because an action must be done. Also, perhaps he is not permitted until nightfall. Rav Amram permits a Tamei Mes right after Tevilah, for he cannot become fully Tahor nowadays. We conclude that he need not immerse. Since he cannot become fully Tahor, he need not become partially Tahor. Perhaps one must totally purify himself from Tum'ah that came from his body. We did not say so for one who is Tamei Sheretz, for it is unreasonable to be more stringent about him than a Tamei Mes.

Rambam (Hilchos Bikurim 5:10): Kohanim may eat Chalah of Chutz la'Aretz even if it is Tamei, unless Tum'ah exudes from the Kohen's body, e.g. a Ba'al Keri, Nidah or Metzora. Therefore, if a young Kohen did not yet see Keri or Nidah, or an adult immersed from Keri, one may separate Chalah for the child.

Rambam (Hilchos She'ar Avos ha'Tum'ah 5:2): Semen of a minor at most nine years old is not Tamei.

Birkei Yosef (OC 457:5): Mahari Lev says that the Rambam refers to being Metamei others, but even a one day old becomes Tamei through Keri. He proved this from the Rambam (Hilchos Terumos 7:9), who says "a minor Kohen who did not yet see Keri." This implies that had he seen, he would be Tamei. The Rambam teaches that even if he is above nine, if he says that he never saw, he is Tahor. Ra'avan says that a one day old is Metamei through Zivah or Keri, like it says in Nidah. This is astounding.

Rambam (Hilchos Mishkav u'Moshav 1:12): The first sighting of Zivah has no Tum'as Mishkav. It is like semen, whether it is from an adult or minor.

Rebuttal (Ra'avad): The first sighting of Zivah of a minor is not Tamei at all, until he is nine years old!

Rosh (3:6): Where there is a minor Kohen who never saw Keri, one may bake Chalah [of Chutz la'Aretz for him on Yom Tov], like it says in Bechoros. Also Bahag brings so from Rav Kohen Tzedek. Even an adult who had an emission, after immersing he is no more stringent than a Tamei Mes, who is permitted. Also, even a Ba'al Keri may eat it through Bitul.

Beis Yosef (OC 457 DH Aval): Also Hagahos Maimoniyos (Hilchos Yom Tov 3:6) says so.

Poskim

Shulchan Aruch (OC 457:2): If one kneads during Pesach, he does not designate Chalah until after it is baked.

Rema: If a minor Kohen never saw Keri, or an adult immersed after his last emission, one may bake Chalah for him on Yom Tov.

Beis Yosef (DH v'Kohen): The Tur connotes that for any minor, the Chazakah is that he never saw Keri. He is believed to say so, even if he is 11 or 12. R. Yerucham wrote "if there is a Kohen less than nine, that Tum'ah never left his body..." This connotes that if he is above nine, the Chazakah is that he saw Keri. This is because we hold that his Bi'ah is Bi'ah.

Magen Avraham (7): The minor must be less than nine. Then, Stam, we say that he never saw Keri. If we know that he saw Keri, even if he is one day old, he is Tamei.

Rebuttal (Chak Yakov 12, Dagul me'Revavah): Nidah 32b and the Rambam explicitly say that Keri of a boy less than nine is not Metamei.

Birkei Yosef: I support the Chak Yakov from the Sifri (Ki Tetzei 255): It expounds that an "Ish" is sent out of two camps due to a nocturnal emission, to exclude a minor. This shows that a minor's semen is not Metamei. The verse discusses a Ba'al Keri, like it says in Pesachim. Also Semag (Lavin 304) says so. The Mishneh l'Melech (Hilchos Bi'as Makdish 3:3) asked why the Rambam omitted this. The Maharit said that the Rambam relied on what he wrote in Hilchos Beis he'Bechirah. In any case the law is true. Also Rabbeinu Hillel on the Sifri says so.

Birkei Yosef: Re'em (Yere'im 304) brings the Sifri, and explains "we exclude a minor, i.e. an adult is not commanded to expel him, according to the opinion that Beis Din must separate minors from Isurim. Here, the Torah exempted Beis Din. The opinion that Beis Din need not separate minors explains that one may bring a minor [who saw Keri] into the Mikdash, even though one may not enter Tamei Kelim. It is a Gezeras ha'Kasuv, even though his Keri is Tamei, like it says in Nidah about Zivah, and Keri is equated to Zivah." I do not understand Re'em. The Ra'avad and Rambam agree that semen of a boy below nine is Tahor! This is clear from Reish Lakish's question.

Mishnah Berurah (20) and Sha'ar ha'Tziyun (31): The Mechaber discusses Chalah of Eretz Yisrael, which Kohanim may not eat nowadays, for they are all Tamei Mes. The Rema discusses Chalah of Chutz la'Aretz. It is forbidden only to one whose Tum'ah came from his body. A Kohen below nine years is not Metamei through Keri (like Chak Yakov and the other Acharonim, unlike the Magen Avraham), and also presumably he never saw Keri. Also an adult who immersed is permitted.

Kaf ha'Chayim (66): Chak Yosef and Mahari Lev supported the Magen Avraham, but the Birkei Yosef and others rejected them.

Gra (DH ha'Lash): Hagahos Maimoniyos and the Mordechai permit even if he did not immerse, because he could immerse. We hold like Rabah, who says Ho'il, like R. Eliezer. The Halachah follows R. Eliezer. Others disagree. We rule like Rabah only regarding Hachanah.

Gra (DH Kohen): The Rema is like the Rif. The other Poskim rule like Tosfos, who permit an adult due to Ho'il. The Rosh and other Poskim permit even if he did not immerse, or even without a Kohen, through Bitul in the majority. We permit only through a Kohen.

Rema (ibid.): Some say that we do not feed Chalah to any Kohen nowadays.

Magen Avraham (9): Nowadays we do not consider a Kohen to be Vadai. Perhaps one of his mothers (i.e. wives of his paternal ancestry) became a Chalalah.

Mishnah Berurah (22): Letter of the law, the Rema holds like the first opinion, that we give it to a minor or an adult who immersed.

Kaf ha'Chayim (73): Mahariyo says that we do not give to a minor Kohen lest he make crumbs, and Takalah will result (Temei'im will eat it), or because Kohanim nowadays are not Muchzakim.