Charles Ellicott Commentary Luke 2:43

"and when they had fulfilled the days, as they were returning, the boy Jesus tarried behind in Jerusalem; and his parents knew it not;" — Luke 2:43 (ASV)

The child Jesus remained behind in Jerusalem. The words do not imply that He intentionally stayed behind. If we consider the history from a human perspective, the likely course of events was this: The Passover Feast lasted seven days. On each of those days, after the first, we may well believe the “child Jesus” was seeking wisdom to do His Father’s work from the appointed teachers who "sat in Moses’ chair." This had become habitual.

He went, as usual, when the Feast was over; but Joseph and Mary, instead of seeking Him there, took for granted that He had started with the other boys of the same age who had come from Nazareth. He was therefore left in the strange city by Himself, finding shelter for the night, probably, in the house where Joseph and Mary had lodged during the feast, and spending the day, as before, in drinking in the wondrous things of God’s Law, and asking questions which showed that He demanded more than traditional or conventional explanations. His question, “Wist ye not...?” implies that they ought to have known where He would be.

Joseph and his mother did not know of it. The better manuscripts read, his parents, the alteration having probably been made in the received text for the same reason as that in Luke 2:33.