What does Luke 2:42 mean?

As Luke points out more than once, Jesus' youth was fundamentally the same as any other child (Luke 2:40, 52), other than a lack of sin (Hebrews 4:15; 1 Peter 2:22).

Just two verses ago, Jesus was described in His infancy (Luke 2:39–40). Scripture records almost nothing about the time between Jesus' birth and the beginning of His public ministry. This is partly a matter of efficiency; the Bible couldn't record every detail of Christ' life without becoming cumbersome (John 21:25). It also might be a question of distraction. As fallible people, we tend to obsess over details that aren't all that important. Had Scripture depicted Jesus' hair, eyes, or height, Christians would have to fight the temptation to treat those as a human ideal (Isaiah 53:2). Children would live under the intense pressure of parents who knew precisely when Jesus learned to walk, to read, and so forth.

The family is heading to Jerusalem as part of a required festival (Deuteronomy 16:16). Since these events were mandatory for all capable Jews, it's likely Jesus, Joseph, and Mary were travelling with a relatively large group. At the age of twelve, Jesus would have been close to His era's concept of adulthood. That His mother and adoptive father don't realize He's still in the temple until a day after they leave is awkward, but hardly negligent.