17 Consider when this was. When the Lord Jesus, as to His Human Nature, was twelve years old (for as to His Divine Nature He is before all times, and without time), He tarried behind them in the temple, and disputed with the elders, and they wondered at His doctrine; and His parents who were returning from Jerusalem sought Him among their company, among those, that is, who were journeying with them, and when they found Him not, they returned in trouble to Jerusalem, and found Him disputing in the temple with the elders, when He was, as I said, twelve years old.
But what wonder? The Word of God is never silent, though it is not always heard. He is found then in the temple, and His mother says to Him, “Why have You thus dealt with us? Your father and I have sought You sorrowing;” and He said, “Did you not know that I must be about My Father's service?” This He said for that the Son of God was in the temple of God, for that temple was not Joseph's, but God's. See, says some one, “He did not allow that He was the Son of Joseph.” Wait, brethren, with a little patience, because of the press of time, that it may be long enough for what I have to say.
When Mary had said, “Your father and I have sought You sorrowing,” He answered, “Did you not know that I must be about My Father's service?” for He would not be their Son in such a sense, as not to be understood to be also the Son of God. For the Son of God He was— ever the Son of God— Creator even of themselves who spoke to Him; but the Son of Man in time; born of a Virgin without the operation of her husband, yet the Son of both parents. Whence prove we this? Already have we proved it by the words of Mary, “Your father and I have sought You sorrowing.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)