A.T. Robertson Commentary Luke 2:42

"And when he was twelve years old, they went up after the custom of the feast;" — Luke 2:42 (ASV)

Twelve years old (ετων δωδεκα). Predicate genitive. Luke does not say that Jesus had not been to Jerusalem before, but at twelve a Jewish boy became a "son of the law" and began to observe the ordinances, putting on the phylacteries as a reminder.

They went up (αναβαινοντων αυτων). Genitive absolute with present active participle, a loose construction here, for the incident narrated took place after they had gone up, not while they were gong up. "On their usual going up" (Plummer).