Let's consider that passage at this time - "And the Child grew and became strong in spirit, filled with wisdom; and the grace of God was upon Him. His parents went to Jerusalem every year at the Feast of the Passover. And when he was twelve years old, they went up to Jerusalem according to the custom of the feast. When they had finished the days, as they returned, the Boy Jesus lingered behind in Jerusalem. And Joseph and His mother did not know it; but supposing Him to have been in the company, they went a day's journey, and sought Him among their relatives and acquaintances. So when they did not find Him, they returned to Jerusalem, seeking Him. Now so it was that after three days they found Him in the temple, sitting in the midst of the teachers, both listening to them and asking them questions. And all who heard Him were astonished at His understanding and answers. So when they saw Him, they were amazed; and His mother said to Him, 'Son, why have You done this to us? Look, your father and I have sought You anxiously.' And He said to them, 'Why did you seek Me? Did you not know that I must be about My Father's business?' But they did not understand the statement which He spoke to them. Then he went down with them and came to Nazareth, and was subject to them, but His mother kept all these things in her heart. And Jesus increased in wisdom and stature, and in favor with God and men."
Although Jesus was divine, He was also human and physically continued to develop as such after birth. Our curiosity makes us wonder about Jesus' life growing up, but the Bible only gives us this brief glimpse into that time frame. The Holy Spirit did not see fit to record any other specifics about Jesus' life until He was about thirty years old (cf. Luke 3:23). Those years were certainly important in His development, but they were not important to the message the gospel writers were seeking to communicate.
Jesus, from age twelve on, would have been required to attend the annual feasts. After this particular feast, Joseph and Mary traveled home with a large group of friends and relatives. They assume that Jesus is "in the company" in spite of the fact that He is not actually with them personally.
After one day's journey and their failure to find Him among their friends and relatives, Joseph and Mary go back to Jerusalem to search for Jesus. Any parent who has temporarily lost a child can easily imagine their feelings at this time.
They eventually found Jesus in the temple. Although He was not formally teaching, He participated by asking and answering questions. "All who heard Him were astonished", certainly because of His age. Even Mary and Joseph were amazed.
Luke 2:49 contains the first recorded statement from Jesus, as He answers the inquiry of His parents about why He didn't leave Jerusalem with them. Interestingly, His first recorded words are about the same Being, the Father, to whom He would direct His last words upon the cross (cf. Luke 23:46). Even from His childhood, Jesus was always mindful of the Father's will, and His need to submit to it. That is why Jesus was surprised by the question His mother asked Him. His answer is not a rebuke but an expression of shock over their anxiety. He doesn't understand why they would be concerned, for He was exactly where they should have expected Him to be; He was in His Father's house, about His Father's business.
Luke 2:51 states that Jesus was an obedient Child (cf. Eph. 6:1-4). This incident in Jerusalem should not be understood as an act of disobedience. After all, He was about His Father's business. This chapter closes with Jesus continuing to develop in all ways - "in wisdom and stature, and in favor with God and men" (2:52).