Each year Jesus’ parents went to Jerusalem for the feast of Passover, and when he was twelve years old, they went up according to festival custom. After they had completed its days, as they were returning, the boy Jesus remained behind in Jerusalem, but his parents did not know it. Thinking that he was in the caravan, they journeyed for a day and looked for him among their relatives and acquaintances, but not finding him, they returned to Jerusalem to look for him.I love this passage, because there's a lot hidden just beneath the surface. At first, this passage seems sort of a strange inclusion on Luke's part, as the only info he provides up about Jesus between age 1 and 30. In fact, it sounds to the casual reader as if Jesus is rebuking His Mother (this was, unfortunately, the mistake which even our priest at the Vigil Mass tonight made in an otherwise decent homily on the importance of tending to our spiritual family, the Church). On the contrary, He's teaching Her, and providing Her an amazing gift.* And the Gospel even makes a note of His obedience to Mary and Joseph towards the end. After all, He's perfect, the Ten Commandments instruct us to honor our father and mother, and Jesus does so. Here, He's honoring both simultaneously, although that may not be immediately clear.
After three days they found him in the temple, sitting in the midst of the teachers, listening to them and asking them questions, and all who heard him were astounded at his understanding and his answers. When his parents saw him, they were astonished, and his mother said to him, “Son, why have you done this to us? Your father and I have been looking for you with great anxiety.” And he said to them, “Why were you looking for me? Did you not know that I must be in my Father’s house?” But they did not understand what he said to them.
He went down with them and came to Nazareth, and was obedient to them; and his mother kept all these things in her heart. And Jesus advanced in wisdom and age and favor before God and man.
Of everyone in the world beside Jesus Himself, His Crucifixion was hardest on His Mom. She knows that Jesus has to die for Her own salvation (cf. Luke 1:47), and so She bears what any of us mindful of the terrible debt He had to pay for us must bear. But She also bears the burden of watching a Son humiliated, tortured, and killed in Her presence. This isn't just common sense, it's suggested in the prophesy of Simeon from Luke 2:34-35. As Simeon prophesies to Mary that Her Son will be "destined to cause the falling and rising of many in Israel, and to be a sign that will be spoken against, so that the thoughts of many hearts will be revealed," he warns Her that "a sword will pierce through your own soul, too."
So what does the twelve-year old Jesus do? He follows the will of His Heavenly Father, and remains in the Temple, knowing that He'll seem to be lost to Her. And note the timing. It's Passover, and He's lost to Her for three days. It's a foretaste of the Passion. I'm speculating here, but it seems to me that the most likely reason is that this pain is a sweet relief for later: that when She watches Her Son die, a seeming failure, on the Passover, She can hope that on the Third Day She can again discover He was simply about His Father's business. This seems to have been the case, since we're told that while "they did not understand what He said to them" at the time, "His Mother kept all these things in her heart." And how does St. Luke know what Mary kept in Her heart? Well, almost certainly because She told him as he was writing his Gospel. And why mention this story? Why would Mary think it important for a Gospel writer to know about this particular incident from His Childhood? I think the answer is obvious enough - She recognized it for the very thing He intended Her to recognize it as: a foretaste of His Passion and Death.
*To preempt an argument I've heard before against the Catholic view of Our Lady, obviously one can be sinless and not omniscient. Catholics don't think Mary knew everything or is God. And even Jesus "advanced in wisdom and age and favor before God and man," so if the sinless and perfect God-Man can still learn a thing or two growing up as a Man on Earth, I think it's safe to say we don't think Mary was beyond instruction, without that disproving anything which we do believe.