twelve years old : when every Jewish boy becomes "a son of the law "If they performed "all things" according to the Law, Joseph had paid the five shekels redemption money (Numbers 3:47; Numbers 3:18; Numbers 3:16), which gave Joseph the legal right to be reckoned the "father", claiming the obedience shown in Luke 2:51. See notes on Luke 2:45, and Luke 3:23, which, thus explain the genealogy there.
Related Commentaries of Luke 2:42
Luke 2:42
42 And when he was twelve years old, they went up to Jerusalem after the custom of the feast.