Gemara

Kesuvos 70a (Mishnah): If Pa'utos (mature children) buy or sell Metaltelim (possessions other than land), it is valid.

(Rafram): If they have an overseer, the sale is invalid. We learn from the Seifa of the Mishnah (69b), 'the actions of a minor are void'.

Question: Perhaps this is only when the property was entrusted to a third party!

Answer: Regarding an adult, the Mishnah said 'the third party does like he was instructed'. Here it says 'the actions of a minor are void' to include other cases (similar to a third party).

Gitin 59a: Rav Yehudah says that children are considered Pa'utos at the age of six or seven. Rav Kahana says that the age is seven or eight. A Beraisa says that it is nine or ten.

They do not argue. It varies, according to the sharpness of the child.

Question: Why was it enacted that Pa'utos can buy and sell Metaltelim?

Answer (R. Yochanan): This was so they can buy Kedai Chayav (life essentials).

(Mar bar Rav Ashi): Such a child can buy and sell Metaltelim whether he is healthy or Shechiv Mera (sick and fears lest he die).

59b (Mishnah): For the sake of Shalom, if a Cheresh, lunatic or minor picked up a (Hefker) Metzi'ah, it is considered theft to take it from him.

64b (Rav Yehudah): If a child knows to discard a rock but keep a nut, he can acquire for himself, but not for others. If one can deposit an item by him and he will return it later, he can acquire for himself and for others.

Shmuel: Both of these children have the same law.

Rav Chisda: He means that either can acquire for himself, but not for others.

65a (Rava): If a child discards a rock but keeps a nut, he can acquire for himself, not for others. Pa'utos can buy and sell Metaltelim.

Bava Basra 156b (Mishnah - R. Eliezer): We may acquire on behalf of a minor, but not on behalf of an adult. (An adult could acquire for himself);

R. Yehoshua says, we may acquire for a minor, all the more so for an adult!

Rishonim

Rif and Rosh (Gitin 27b and 5:19): Chachamim enacted that Pa'utos can buy and sell for their needs. Pa'utos are children from six to 10, depending on their astuteness.

Rif and Rosh (30b and 7:10): If a child discards a rock but keeps a nut, he can acquire for himself, not for others. Pa'utos can buy and sell Metaltelim.

Ran (DH Omar): Rava gives a Shi'ur for a child to acquire for himself. If a minor picked up a Metzi'ah, one may not take it due to Darchei Shalom (59a). Letter of the law, he did not acquire! Rava says that he can acquire through Da'as Acheres Makneh (when an adult gives to him). However, Bava Basra 156 says that a minor cannot acquire even through Da'as Acheres Makneh! Here we refer to Metaltelim, and only when the item comes to his hand. A minor cannot acquire through Agav or Chalipin, for he does not understand acquisitions. Some explain that Bava Basra discusses a child below the age to acquire through Da'as Acheres Makneh.

Rebuttal (Shach CM 243:6): The Gemara did not distinguish between whether or not Da'as Acheres Makneh! Rather, the Mishnah (59a) discusses a child too young to acquire. The Yerushalmi says so. The Ran himself says that Bava Basra discusses such a child, even though it did not specify.

Defense (Magen Sha'ul, Leshonos ha'Rambam 36:3, cited in R. Akiva Eiger on the Shach): The Yerushalmi teaches a third level of maturity, when a child knows to store away a nut. Chachamim agree to R. Yosi that taking such a child's Metzi'ah is proper theft mid'Rabanan.

Rif and Rosh (6:23): The purchase and sale of Pa'utos are valid. However, 'the actions of a minor are void', i.e. whether the property was entrusted to a third party, or if there is an overseer.

Rambam (Hilchos Mechirah 29:1): Mid'Oraisa, a child cannot buy or sell. Chachamim enacted that he can buy and sell Metaltelim Kedai Chayav.

Magid Mishneh: One may not take the Metzi'ah of a minor due to Darchei Shalom. Mid'Oraisa, he cannot acquire. Chachamim enacted that Pa'utos can sell Kedai Chayav. If not for this need, they could not sell.

Rambam (6): Until six yearshe cannotbe Makneh anything to others. From six until adulthood we check him. If he understands buying and selling, his purchase, sale and giftof Metaltelim are valid, for big and small matters.

Rambam (7): This refers to a child without an overseer. If he has an overseer, his actions are void even in Metaltelim, unless the overseer approves. If he approves, the child's purchase, sale or gift of Metaltelim is valid.

Rambam (8): We check a minor's proficiency about buying and selling until he matures. Some minors are clever and know at the age of seven. Some do not know even when they are 13. If he knows, and he has no overseer, if he bought or sold Metaltelim and erred, he is like an adult. An error less than a sixth is pardoned. An error of a sixth is returned. An error more than a sixth voids the sale. I say that his purchase and sale are valid only if he or the buyer did Meshichah. If he paid money for a purchase and retracted, he does not receive a Mi she'Pora (a curse for one who retracts). If an adult gave money for a purchase to a minor and retracted, he receives a Mi she'Pora.

Rebuttal (Ra'avad): The Gemara says that orphans (i.e. minors) cannot pardon.

Defense (Magid Mishneh): If this is true, the Gemara should have explained so. It merely says that his error is like an adult's! Can we say that a minor's overcharge is pardoned, but an adult's must be returned?! (If so, no one will want to deal with him!) Rather, just like Chachamim enacted that he can buy Kedei Chayav, they enacted that he is like an adult regarding errors.

Rambam (Hilchos Zechiyah u'Matanah 4:7): If a child knows to discard a rock but keep a nut, he can acquire for himself, but not for others. Before this he cannot acquire even for himself.

Poskim

Shulchan Aruch (CM 235:1): Until six years he cannot Makneh anything to others.

SMA (2): The minor can retract, for Ein Chavin l'Adam she'Lo b'Fanav (we cannot obligate someone in his absence. A minor lacks Da'as, so it is as if he is not here.) The seller cannot retract.

Shulchan Aruch (ibid.): From six until adulthood, if he is proficient about buying and selling (Rema - he was tested and found to know; after 10, it suffices that he is not a lunatic), his purchase, sale and gift are valid. This is for big and small matters, whether he was healthy or Shechiv Mera. This is an enactment mid'Rabanan regarding Metaltelim.

Tur: The Ramah holds that past 10 we assume that a child is proficient, unless we learn otherwise. The Rosh holds that past 10 he is surely proficient, unless he is a lunatic.

SMA (3): The Rema should not have brought the Rosh's opinion to explain the Mechaber, who cites the Rambam, who holds like the Ramah!

Gra (2): The Rambam holds that there are different Shi'urim for the start of Pa'utos, but Pa'utos extends until adulthood. We must check the proficiency of a minor until adulthood.

Rema: Some say that since it is only an enactment, if they acted improperly, e.g. boys sold property of a small estate, in which case the girls merit the property (to be fed from it), the sale is void.

Shulchan Aruch (2): This discusses a child without an overseer. If he has an overseer, his actions are void, even in Metaltelim, unless the overseer approves. If he approves, the child's purchase, sale or gift of Metaltelim is valid.

Beis Yosef (DH u'Mah she'Chosav ba'Meh): Rashi says that this is whether the overseer was appointed by his father or by Beis Din. The Rivash (468) says that if the orphans rely on a Ba'al ha'Bayis who manages their property, the same applies. We need not enact for Kedei Chayav in this case.

Rema: If he eats by a Ba'al ha'Bayis, the Ba'al ha'Bayis is like an overseer. Once the child matures, his purchase and sale are valid. Even though the overseer must fulfill the Mes' command, and he may not give the property to the heir or buyer until the time he was told, the sale is valid. If the buyer seized the property, we do not take it from him.

Shulchan Aruch (243:15): If a child knows to discard a rock but keep a nut, he can acquire for himself.

Rema: This is only with Da'as Acheres Makneh, but not for a Metzi'ah.

Shulchan Aruch (ibid.): He cannot acquire for others. Before this age, he cannot acquire for himself or others.