I read the excellent piece http://dinonline.org/2015/11/23/bar-mitzvah-maturity-of-body-or-mind/
While a child over 13 without being checked or without facial hair can’t be motzee others in a Torah mitzvah or write tefillin, for example, is he allowed/do we allow him to write his own mezuzah or blacken his own retzuos if he claims that he has 2 sa’aros? (I’ve not heard of bachurim asking adults to touch up their tefillin. In fact, we see many BM boys with tefillin paint markers or bottles in their zekel.
Furthermore, what’s the geder of having facial hair? What if he had this facial hair at bar mitzvah?
Kindly include mekoros including poskei zmaneinu.
If the boy claims that he has two hairs we believe him, and we consider him a gadol for everything. The reason for this is that since the majority of children have grown two hairs at that point, the majority is on his side so we can believe him. Another reason is since it is something that can be verified, we don’t assume that he is lying.
The Mishna Berura talks about a different issue, if he didn’t claim anything, but he is over bar mitzvah may he fix things for himself. He discusses this in two places, he says that a bar mitzvah boy may paint his own teffilin, and he may make his own tzitzis. His reasoning is that even if he is now only required to put the teffilin on m’drabonon, but m’drabonon he is also considered a gadol. (He doesn’t address the issue of what will he do when he becomes a real Gadol, but he doesn’t seem to be concerned with this.)
It isn’t dependant on facial hair per say, rather on body hair, which comes much before facial hair does. Practically since these areas are covered, we wouldn’t know it until we see facial hairs.
Regarding the “geder of facial hair” see Rambam Hilchos Ishus 2-15, 16, 17. If he has these hairs before his 12th birthday they don’t count, however afterwards, when he enters his thirteenth year of life, ( after his 12th birthday) they do count, (see Rambam ibid 9,10).
Kaf Hachaim O:CH 55-58, Noda B’yehudah E:H 1- 41, Imrei Bina CH:M 49 . Siddur R’ Y. Emden Hilchos Kaddish 9, disagrees with this, that we don’t believe him.
Biur Halcha 14-1 D:H l’hatzrich, 39-2 D:H b’chol.