Twelve years old (etwn dwdeka). 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 (anabainontwn autwn). 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).