When [Jesus] was twelve years old, [Jesus, Mary, and Joseph] went up to the festival [in Jerusalem], according to the custom. After the festival was over, while they were returning home, the boy Jesus stayed behind in Jerusalem, but [Mary and Joseph] were unaware of it. Thinking he was in their company, they traveled on for a day. Then they began looking for him among their relatives and friends. When they did not find him, they went back to Jerusalem to look for him. After three days they found him in the temple courts, sitting among the teachers, listening to them and asking them questions. Everyone who heard him was amazed at his understanding and his answers. When his parents saw him, they were astonished. His mother said to him, “Son, why have you treated us like this? Your father and I have been anxiously searching for you.”
Why were you searching for me?” he asked. “Didn’t you know I had to be in my Father’s house?” But they did not understand what he was saying to them.
Then he went down to Nazareth with them and was obedient to them. But his mother treasured all these things in her heart. And Jesus grew in wisdom and stature, and in favor with God and man. (Luke 2:41-52)
Now Jesus himself was about thirty years old when he began his ministry. (Luke 3:23)
Much has been made about Jesus’s supposed “lost” years. (What happened to him when he was a teenager? What happened to him when he was a young adult? What happened to him during those years?)
That said, much has been speculated about what happened to him during those supposed “lost” years (at times, people have suggested that he traveled to Egypt and India, lived with the Essenes, traveled to Persia and Atlantis, etc.).
In order to answer this question, we need to consider how the ancients defined what we would call a “biography”.
(n.b., our modern word “biography” is derived from two Greek terms: bios, meaning “life”, and graphia, meaning “to write”. It goes without saying that a biography is a written account of a person’s life.)
Getting back to what the ancients regarded as a biography, they had a completely different concept of it than we do today. Most of us today tend to define a biography as a written, sequential listing of the events of a person’s life (“This event happened … after that, this event, this event, this event, and this event happened, followed by this event”).
The ancients, on the other hand, were much more interested in the character of a person (What did they believe? How did they live? What did they teach, if anything? What made them tick?).
This is why the four Gospels (Matthew, Mark, Luke, and John) have long stood as biographies of Christ, even though none of them agree with each other (Christ names Peter the head of the church in the gospel of Matthew, but not in Mark, Luke, or John. Christ re-instates Peter as the head of the church — this after Peter denied knowing Christ three times prior to his crucifixion — in the gospel of John, but not in Matthew, Mark, and Luke. Matthew and Luke contain nativity narratives — both of which differ significantly — while Mark and John do not. All of the gospels contain wildly differing accounts of Jesus’s crucifixion and resurrection.)
The short answer to the question that I posed above (What happened during Jesus’s “lost” years?) is thus: nothing that the ancients would’ve found particularly interesting. In all likelihood, he grew up living the typical life of a Jewish boy of that era, working alongside his father Joseph and going to the synagogue, where he learned the Hebrew scriptures that he himself — as a grown man in his early thirties — would someday preach.