Parshas Zachor
Question
Why was the Bar Mitzva boy not allowed to read Parshas Zachor?
Answer
Shalom!
Thank you for your question.
The reason why Bar Mitzva boys generally do not read the “Zachor” portion even if their Bar Mitzva is being observed on that Shabbos is because the Parshas Zachor, the special Torah reading to remember the treachery of Amalek that is read on the Shabbos before Purim, is considered to be a Torah obligation.
Contrary to widespread misconception, it is not enough for a boy to be 13 years odl to be considered an adult, but in addition to his age, he must have signs of physical maturity. As a general rule, such a thing is not checked, but merely assumed to be present when a boy is 13. Nevertheless, for matters of a Torah nature we are more strict. In such instances a make must be 18 years old to be unquestionably considered an adult in Jewish law or if he grows facial hair.
As such, it is indeed preferable that a Bar Mitzva boy or any young man not read the Zachor portion.
Source
OC 685:7; Magen Avraham 39:1