Shevuos Chapter 6, Mishnah 4. This Mishnah discusses the legal standing a person needs in order to obligate another in an oath, and who lacks the power to impose an oath at all.
"Ein nishba'in al ta'anas cheresh, shoteh v'katan" - one does not swear on the claim of a deaf-mute, an incompetent person, or a minor:
A defendant does not become obligated in an oath when the claim is made by any of these three:
Cheresh - a mute.
Shoteh - one who is not of sound mind.
Katan - a minor.
The reason is that they have no legal standing, and they do not have the power to make a legal claim. It is true that the Rabbis, in the time of the Gemara, instituted the oath of hesses, and that oath would apply here; but in the time of the Mishnah it did not yet exist.
"Ein mashbi'in es hakatan" - one does not administer an oath to a minor:
Even if a claim is made against a minor, he does not swear, because he does not grasp the seriousness of the matter, the meaning of an oath, or the punishment that comes to one who swears falsely.
"Aval nishba'in l'katan" - but one does swear to a minor:
If someone comes to collect a debt that the minor's father owed him, out of the estate that the son inherited from his father, the halachah is that he must swear in order to collect the debt.
Swearing to hekdesh:
Similarly, if a person dedicated his property to the Temple treasury, and a creditor comes to collect from it because he had a prior lien on these items before they were dedicated, he must swear that these items are indeed owed to him, in order to collect from hekdesh.
In summary: In this Mishnah we learned that a deaf-mute, an incompetent person, and a minor cannot impose an oath, because they have no legal standing to make a claim (and only by rabbinic enactment was the oath of hesses instituted), and that a minor is not made to swear because he does not understand the meaning of an oath or its punishment. On the other hand, one does swear to a minor - when coming to collect a debt from the estate the minor inherited - and one swears to hekdesh as well, when a creditor's lien preceded the dedication of the property.