‘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 did not know it.'
Every Jewish boy came of age at thirteen from which point on he was looked on as a responsible adult and expected to fulfil his religious responsibilities, becoming ‘a son of the Law'. Thus the Rabbis recommended that boys who were approaching that age be brought to the feasts so that they could become acquainted with the atmosphere and with what went on.
So when Jesus was twelve His parents took Him up to the Feast of the Passover, and once the seven days of unleavened bread were over they set off to return to Nazareth with a large group of Galileans.
What happened appears to indicate that on the pilgrimage to Jerusalem, on which would also be all their relatives, it was quite normal during the festivities for boys of twelve, who were seen as almost mature, to go around together enjoying the festival (compare modern older teen-agers who would not want to be tied to their parents), and when hungry or tired, to stay with one or other of their relatives whose son(s) would be one of them. Then, of course, when it was time to go back home, whoever they were with could be expected to see that they were included in the caravan. This is really the only explanation as to why Jesus had not been missed, and why they set off without Him. They had had confidence in Him that He would not get up to mischief, and in their relatives that whoever He was staying with would ensure that He was properly looked after and would set off back for Galilee with them. Probably in previous years this had worked very well. What they had not taken into account, and what Jesus considered that they ought to have taken into account, was that now that He was almost ‘of age' it was necessary for Him to go to His Father's house to learn of Him.
In such caravans the men would often walk together in a large group, while the women went ahead in front, and this may well have been why they did not ask each other where Jesus was. Joseph may have thought that Jesus had joined up with Mary, and Mary may have thought that He had joined up with Joseph. Or both may have been satisfied that He would be with relatives. But although they did not know it Jesus had lingered in Jerusalem, for He had gone to the Temple and was listening to the great teachers. It seems that He just assumed that when His parents wanted Him they would come for Him there because in His view ‘they should know that He was there'.
But we may ask as to whether a boy, even though a ‘mature' boy (pais), would really remain in the Temple day and night for three days without going back to His parents. There could only be two reasons why this was feasible; either it was normal for boys of his age to go about with boys of their own age during such festivals, sleeping where they liked and obtaining food from different relatives who would be there, or even from generous pilgrims, so that He did not see this as unusual, or because He had in fact tried to go back to His parents, only to discover that they had disappeared. This would leave Him having to find something to do until they came back for Him. Being what He was He thus went back to the Temple confident that His Father would watch over Him.