In the post – Josiah by His Numbers I listed Josiah’s life and the numbers that are given in Scripture.  The numbers are found in both 2 Kings and 2 Chronicles, God repeated Himself, so you know they are important.  There is much written on numbers, there occurrences, uses, and meanings on the internet, be careful as some sites don’t always glorify Jesus.

• Seven is a number showing completion, it also carries covenant context. Josiah took seven years to clean the idols and other pagan influences of Judah and Israel.  it probably took that long because the people and leaders seemed pretty comfortable with idolatry and keeping the Lord as a figurehead.
• Eight – Eight is referred to the number of new beginnings, I have often felt this is because of Genesis and the days of creation. In Josiah’s life, eight signals a “new” of something that had or should have been going on. His “eights” are the beginning of his reign and him starting to seek God. The “seeking God” is impressive because he had no copy of the Law, it was given to him ten years later (also the number of Commandments).
• Twelve (apostles) is associated with leadership. His twelfth years as a ruler is when he started taking control and clearing out pagan worship.
• Thirteen (see my post of 13) is the start of something new. Not recycling or restarting, but brand new.  Jeremiah started his ministry in the 13^th year of Josiah’s reign, and they celebrated the Passover (implied) until Josiah died.
• Sixteen – In my post on sixteen, it seems that it was associated with the “eye of God” keeping watch. Josiah starting to seek God at sixteen fits that thought well, God was keeping his eye on him.
• Eighteen is a number with a lot associated it with (See Eighteen in the Bible), but it representing fullness makes the most sense to me. In Josiah’s life, it was the eighteenth year when his campaign to up Judah came to its “fullness” with the finding of the Law and Passover celebrated properly.
• Twenty –Josiah’s waited until the age of twenty to start his battle against pagan influences.  This corresponds with two themes of twenty in the Bible.  Jacob waited/served twenty years to get his wives, children, and flocks.  Young men entered military service at twenty.