Luke 2:41-51 ESV
Now his parents went to Jerusalem every year at the Feast of the Passover.
And when he was twelve years old, they went up according to custom.
And when the feast was ended, as they were returning, the boy Jesus stayed behind in Jerusalem. His parents did not know it,
but supposing him to be in the group they went a day's journey, but then they began to search for him among their relatives and acquaintances,
and when they did not find him, they returned to Jerusalem, searching for him.
After three days they found him in the temple, sitting among the teachers, listening to them and asking them questions.
And all who heard him were amazed at his understanding and his answers.
And when his parents saw him, they were astonished. And his mother said to him, “Son, why have you treated us so? Behold, your father and I have been searching for you in great distress.”
And he said to them, “Why were you looking for me? Did you not know that I must be in my Father's house?”
And they did not understand the saying that he spoke to them.
And he went down with them and came to Nazareth and was submissive to them. And his mother treasured up all these things in her heart.
Jesus was twelve years old and had reached the age of accountability, when He was of spiritual maturity and able to take responsibility for His actions.
After three days of searching for Him back in Jerusalem, they find Him in the temple sitting among the teachers conversing with them in a way that amazed all those hearing Him speak. The three days of searching may remind us of the three days before He would rise from the dead after His crucifixion. His ministry to humanity would there after be of a different nature, not unlike this change of perspective.
Mary tells Him she and His father had been worried sick being unable to find Him. Jesus replies that He is in His Father's house. They did not understand what He was saying. Jesus was telling them He knew His true Father and it was His Father's business that He must tend to. Jesus returns home with Mary and Joseph as their son, but now He begins preparing to fulfill His calling as the Son of God.
Mary would remember all these things and treasure them in her heart, but the full implications of what just took place would not be understood until later.