Image from the Winchester Bible, showing the seven ages within the opening letter "I" of the book of Genesis. This image is the final age, the Last Judgement. [Image from the Wikipedia 'Six Ages of the World' page, public domain, via Wikimedia Commons.]
The number six (6) comes up at least twice in the movie: In the pier scene, Will tells Molly that "We [the investigators] got about six days until the next full moon." Later, Will angrily accuses Crawford of bringing him into the Tooth Fairy case "knowing God damn well" that he'd imagine "families three, four, five, and six." These mentions of the number 6 refer to the Six Ages of the World.
The Six Ages of the World is a Christian historical periodization outline first written about by Saint Augustine circa 400 AD. It is based along Christian religious events, from the birth of Adam to the events of Revelation. The six ages of history, with each age lasting approximately 1000 years, were widely believed and in use throughout the Middle Ages, and until the Enlightenment, the writing of history was largely the filling out of all or some part of this outline.
The outline accounts for Seven Ages, just as there are seven days of the week, with the Seventh Age being eternal rest after the Final Judgement and End Times, just as the seventh day of the week is reserved for rest. It was normally called the Six Ages of the World because they were the ages of the world, of history, while the Seventh Age was not of this world and lasting forever. The Six Ages are best described in the words of Saint Augustine, found in De catechizandis rudibus (On the catechizing of the uninstructed), Chapter 22:
1) The First Age: "The first is from the beginning of the human race, that is, from Adam, who was the first man that was made, down to Noah, who constructed the ark at the time of the flood."
2) The Second Age: "[E]xtends from that period on to Abraham, who was called the father indeed of all nations.."
3) The Third Age: "For the third age extends from Abraham on to David the king."
4) The Fourth Age: "The fourth from David on to that captivity whereby the people of God passed over into Babylonia."
5) The Fifth Age: "The fifth from that transmigration down to the advent of our Lord Jesus Christ."
6) The Sixth Age: "With His [Jesus Christ's] coming the sixth age has entered on its process."
Augustine was not the first to conceive of the Six Ages, which had its roots in the Jewish tradition, but he was the first to write about it with authority. The theory originates from a passage in 2 Peter: "But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day." [2 Peter 3:8, New International Version] From this it was taken to mean that mankind would live through six 1,000 year periods (or "days"), with the seventh being eternity in heaven.
Christian scholars believed it was possible to determine how long man had been alive, starting with Adam, by counting forward how long each generation had lived up to the time of Jesus, based on the ages recorded in the bible. While the exact age of the Earth was a matter of biblical interpretive debate, it was generally agreed man was somewhere in the last and final thousand years, the Sixth Age, and the final Seventh Age could happen at any time. The world was seen as an old place, and the future would be much shorter than the past; a common image was of the world growing old.
The Ages reflect the seven days of creation, of which the last day is the rest of the Sabbath, illustrating the human journey to find eternal rest with God, a common Christian narrative.[a]
a. Wikipedia, 'Six Ages of the World'. Web, n.d. URL = https://en.wikipedia.org/wiki/Six_Ages_of_the_World.
