This 15th-century page from a Book of Hours
shows the typical medieval composition of treatments on subject.
The episode is described in Luke 2:41-52. Jesus at the age of twelve accompanies Mary, Joseph and a large group of their relatives and friends to Jerusalem on pilgrimage, “according to the custom” - that is, Passover. On the day of their return, Jesus "lingered" in the Temple, but Mary and Joseph thought that he was among their group. Mary and Joseph headed back home and after a day of travel realised Jesus was missing, so they returned to Jerusalem, finding Jesus three days later.
He was found in The Temple in discussion with the elders. They were amazed at his learning, especially given his young age. When admonished by Mary, Jesus replied:
“How is it that you sought me? Did you not know that I must be in my Father’s house?”
The story was slightly elaborated in later literature, such as the apocryphal 2nd-century Infancy Gospel of Thomas (19:1-12). The losing of Jesus is the third of the Seven Sorrows of Mary, and the Finding in the Temple is the fifth Joyful Mystery of the Rosary.