The Mishna teaches that the eiruv is not valid when it is entrusted to the hands of an irresponsible person, like a heresh (a deaf-mute person), a shoteh (an imbecile) or a katan (a minor). The Gemara is surprised that the eiruv of a minor is invalid, since Rav Huna – an amora who cannot argue with the Mishna – rules that a katan can collect the eiruv! The Gemara explains that the two rulings refer to different cases. Rav Huna, who permits the minor’s eiruv, is talking about eruvei ḥatzeirot, the eruv that permits carrying in an adjoining courtyard, which has been the focus of our massekhet up to this point. The Mishna that does not accept the katan’s eiruv is discussing eruvei teḥumin, the eiruv that permits someone to travel beyond the 2000-ama boundary surrounding his community by establishing his place of residence for Shabbat at the edge of the boundary.
The Gemara does not explain why the rules should differ between these two different types of eiruv, and many different suggestions are raised.
According to Rashi, the eiruv that allows the householders to carry in a common courtyard is a formality, since the houses are joined in any case. Eiruvei teḥumin, on the other hand, demands establishing a new living space for Shabbat, which the minor is unable to accomplish.
Tosafot see the difference as being based in the source of the law. Eiruvei te%humin is based on a biblical passage, while eiruvei ḥatzeirot are solely of rabbinic origin.
Rabbenu Yehonatan argues that, in the case of eiruvei teḥumin, the person who establishes the eiruv must state explicitly, “So-and-so is establishing this place as his residence for Shabbat,” which a minor is not trusted to do. The eiruv ḥatzeirot, on the other hand, is a simple delivery, for which the katan can be relied upon.
The explanation given by the Jerusalem Talmud is that the purpose of eiruvei ḥatzeirot in a place where houses are, in any case, closely connected to one another is simply to encourage a sense of community and brotherly love, so we are not overly concerned about who establishes the eiruv.