Why is the custom in many shuls that a bar mitzvah boy not read Parshas Zachor?

Shulchan Aruch (OC 685:7) writes that there are opinions that Parshas Zachor, the special Torah reading to remember the treachery of Amalek that will be read this Shabbos, is a Torah obligation. Only a male adult who is obligated in this mitzvah may read aloud this Torah portion and fulfill the obligation for all those that listen. Although ordinarily, we rely on the assumption that a bar-mitzvah boy has reached physical maturity and is obligated in all mitzvos, when it comes to exempting others in a Torah obligation we are more strict. The Magen Avrohom (39:1) writes that once a boy reaches age eighteen, he can be assumed to be physically mature. At that point he can be relied upon to fulfill Torah obligations, but not before. Therefore, it is preferable that a young man should not read Parshas Zachor. However, some poskim are lenient especially if it might otherwise cause embarrassment or division. See for example Teshuvos P’as Sod’chah (I:63) and Likutei Teshuvos V’hanhagos (Purim, pg. 70).

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