What should a young man do about the excess money he took from a restaurant years ago, knowing that the restaurant owner traveled and his whereabouts are unknown? And does the matter differ based on the value of the money over time?

Whoever takes the right of another is obligated to return it to them. If, after searching for the owner, they cannot find them, they should give the money in charity on their behalf. There is no difference between taking a right before or after reaching the age of accountability in terms of the obligation to return it, except that a person who has reached the age of accountability must also repent in addition to returning the right.