Luke 2:42

And when he was twelve years old, they went up after the custom of the feast; and when they had fulfilled the days, as they were returning, the boy Jesus tarried behind in Jerusalem; and his parents knew it not.

All Jewish adult males were required to attend the passover; and it was usually observed by the entire families of all the people who were physically able to make the journey. There is nothing here of the Bar-Mitzvah service for Jewish boys entering their thirteenth year, although the age of Jesus is certain to bring speculations about it. Caravans of people attending the great feast traveled in companies; and it was quite easy for Jesus to "get lost" on the return journey. Any twelve-year-old boy would have known how to do that!