The Holy Innocents

Matthew 2:16-18 has the story (and References Jeremiah 31:15):

Then Herod, when he saw that he had been tricked by the wise men, became furious, and he sent and killed all the male children in Bethlehem and in all that region who were two years old or under, according to the time that he had ascertained from the wise men. Then was fulfilled what was spoken by the prophet Jeremiah:

 “A voice was heard in Ramah,
    weeping and loud lamentation,
Rachel weeping for her children;
    she refused to be comforted, because they are no more.”

While there is no account of this event outside of the Bible, it is consistent with the actions of Herod. Also, no one knows how many children were killed, but it has been estimated that Bethlehem was a town of about one thousand, so the number of infants two years old and younger was probably around twenty.

The Magi came at some point after Mary, Joseph, and Jesus were no longer staying in the manger, but in a house. Also, this is after Jesus had been taken to the temple, so he was more than eight days old. Herod was probably conservative age-wise by killing all kids under two, so Jesus was younger than two years old.