Why doesn’t a bar mitzvah boy have to say shechiyanu for every mitzvah he does for the first time after his bar mitzvah?
Answer:
Some authorities write that one must make a shehecheyanu the first time one performs a specific mitzvah (see Rema, Yoreh De’ah 28:2; Taz, Orach Chaim 22).
However, for mitzvos that a boy has been performing his whole life, there is no cause for a shehecheyanu upon performing them again after his bar mitzvah, because the mitzvah does not involve the joy of doing something for the first time.
Best wishes.
Sources:
See Adnei Paz, Yoreh De’ah 67, in great detail.
