Christ, whose glory filled the skies
Lift up your heads, ye mighty gates
Breathe on me, breathe of God
There's a song in the air
Blessed, O my soul! The living God.
Thanks we give and adoration
 And Jesus increased in wisdom and in stature and in favor with God and man.
(Luke 2:41-52 ESV)
This is the only account of Jesus as a child.
None of the other gospels do this... So some folks think this story isn't true, that Luke added it just to satisfy people. A bit later than Luke wrote this several gospels of Jesus as a child were written but they have all been proven to not be true. So some people think this is like that.
But this story is not filled with fantastical events. This is just an ordinary account of Jesus and his family.
This line "did you not know I must be about my father's business?"
Jesus family treated Jesus just as all Jewish families handled their children. Circumcision, teaching at temple, travelling to Jerusalem. The twelfth year is an important one.... It's their last year as a child. This last year was one to be trained in religious duties as a man
So Jesus was taken to Jerusalem to learn about the passover.
They travelled in a group as it was the common and safe thing to do. Not worrying about him as it was out of character for them to have to worry about him. They searched for him anxiously.
Then they find him calmly sitting in the temple asking and answering questions. People were amazed at his answers. His parents were also amazed.
They had forgotten who Jesus really was.
Mary asked: why have you treated us so?
It is easy for us to forget who Jesus truly is.
"How is it that you sought me. Did you not know I would be in my father's house and about his business? "
In essence he was saying.... Mom.... You should have known I would be here. You shouldn't have been anxious about it at all.
He didn't stay to worry his parents but to be doing what he was called to do.
Hr reminded his parents who he truly was.
Jesus had to learn and grow in the knowledge of God. He had to gain wisdom.
Mary and Joseph didn't truly understand what Jesus said. We are no different.... We don't always understand Jesus and the things he does either.
This Jesus is the same. Regardless of our understanding. He is ALWAYS about his father's business. We can rest secure in that knowledge. He works even now about his father's business in you.