In the biography of Rabbi Aryeh Levin, zt'l, "A Tzaddik in Our Time," Raz, Simcha (Feldheim 1976), p. 76, I found a surprising statement: "When a certain orphan boy became bar-mitzvah (on his twelfth birthday rather than his thirteenth, because he was an orphan), Reb Aryeh went to the family's celebration of the happy event...."
I never heard of such a thing. What source brings this halacha (or minhag) down?