Q. Kevod Harav Shlit"a, If a boy under the age of bar mitzvah completes an entire masechta and understands it. Should he also recite the kadish upon completion of the siyum?
A. The common custom is that the kaddish after a siyum is recited by someone who already recites or has recited kaddish. However, Horav Shlomo Miller's Shlit'a opinion is that the not Bar Mitzva yet bochur, who completed an entire masechta may recite the kadish after saying the siyum, since that kadish is not considered a kadish yosom, that should be recited by ones who have lost a close dear one.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit'a