Tosfos asks: Since a minor in not a “bar kerisus,” he is incapable of divorcing his own wife, he therefore should be disqualified from writing a get for someone else!?
Tosfos answers: Since he will eventually grow into an adult, he is not regarded as someone who is incapable of divorcing his wife, and therefore, he is still eligible to write a get.
The Noda b’Yehudah (O”C §1) asks: Why is the halacha of writing a get different than the halacha of writing tefillin? A minor is excluded from writing tefillin, since he is not a “bar keshirah,” he is not obligated in the mitzvah of tefillin. Why don’t we say that since he will be obligated in the mitzvah when he becomes an adult, he should be eligible to write tefillin?
Reb Elchonon Wasserman answers: That which we say that someone who is not a “bar kerisus” cannot write a get is only if he is excluded inherently from the subject of divorce. A minor, however, cannot issue a divorce, not because he is excluded, but rather, it is because he is incapable of marrying. He, therefore, can still be regarded as a “bar kerisus,” and can therefore be eligible to write a get. However, with respect of tefillin, a minor is excluded from the obligation of tefillin, and therefore, he is not considered a “bar keshirah,” and is therefore not disqualified from writing tefillin.