8 year olds

2 Kings 22:1 (also mentioned in 2 Chronicles 34:1)
Josiah was eight years old when he began to reign, and he reigned thirty and one years in Jerusalem. And his mother's name was Jedidah, the daughter of Adaiah of Boscath.
2 Chronicles 36:9
Jehoiachin was eight years old when he began to reign, and he reigned three months and ten days in Jerusalem: and he did that which was evil in the sight of the LORD.

William F. Buckley, Jr. - was 8 years old when he wrote to the king of England, demanding payment of the British war debt.