Q. Re- question 2403 above. What happens if the minor is going to become bar mitzva during the shloshim of his father. Is he then obliged to sit shiva or keep the other traditions of avelut after he become bar mitzva?
A. Shulchan Aruch (Y.D. 396: 3) Chochmas Odom (168: 6), Aruch Hashulchan (396: 5) and others rule that even if the minor becomes bar mitzva during the shiva, he does not have to comply with the halachos of avelus, Shach (ibid. 3) quotes Bach and Maharam, that if he became of age during the shloshim days, he keeps shiva and shloshim after the day of his bar mitzva. Taz (ibid. 2) disagrees.
Some maintain that he should keep the traditions that apply after the shloishim until the end of the twelve months.
Horav Shlomo Miller’s Shlit’a opinion is that even if the minor is going to become bar mitzva during the shloshim of one of his parents, there is no need to keep the traditions of avelus.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a