QUESTION: Rashi DH Isura explains that the Rabbanan cannot place any
obligation (e.g., the obligation to fulfill a vow) upon a minor. Rashi's
statement is logically very sound, since the minor is not obligated to
accept upon himself their Gezeiros any more than he is obligated in any
other Mitzvos of the Torah.
RASHI in Berachos 48a (DH Ad she'Yochal) adds, based on this premise, that
even the Rabbinical obligation for a minor to perform, or "practice,"
Mitzvos ("Chinuch") is not incumbent upon the minor. Rather, it is the
*father's* obligation to see to it that his son performs Mitzvos. RAMBAN (in
Milchamos, Berachos 20b) explains that for this reason, a minor may not
recite Birchas ha'Mazon for a person over the age of Bar Mitzvah even if
that person ate less than a k'Zayis of bread (and is obligated in Birchas
ha'Mazon only mid'Rabanan). Although the rule normally is that one who is
obligated in a Mitzvah mid'Rabanan may exempt another person who is also
obligated mid'Rabanan in that Mitzvah, in this case that rule will not
apply. A minor is not obligated in Birchas ha'Mazon even mid'Rabanan; his
*father* is obligated to see to it that the child recites Birchas ha'Mazon.
Why, then, does RASHI himself (in Berachos 20b DH Shiura) rule that a minor
*may* recite Birchas ha'Mazon for a person over the age of Bar Mitzvah if
that person ate less than a k'Zayis of bread! This is the opinion of Tosfos
(Berachos 15a DH v'Rebbi Yehudah; 48a DH Ad) and the Halachic ruling (Orach
Chayim 186:2) as well.
ANSWER: Rashi and Tosfos understood that although a minor is not obligated
*at all* in Birchas ha'Mazon in his own right, nevertheless, the Rabanan who
decreed that a person who ate less than a k'Zayis must recite Birchas
ha'Mazon enacted in their decree that if a minor recites Birchas ha'Mazon
for one who ate less than a k'Zayis, it will suffice. The reason they
enacted their decree in such a manner was in order to further the cause of
Chinuch, by making it *look* to the minor as though he is indeed obligated
in Mitzvos in his own right, so that he should regard his obligation in
Mitzvos with austerity. (M. Kornfeld)