In Doctrine and Covenants 68:27, the age of accountability is clearly defined as being eight years.
27 And their children shall be baptized for the remission of their sins when eight years old, and receive the laying on of the hands.
Section 68 was given November 1831, and is the earliest source for the age of eight as age of accountability that I am aware of. However it is clear from Doctrine and Covenants 18 and 20 (and the Book of Mormon), that the concept of "age of accountability" was known from the foundation of the church in 1830.
What was the "age of accountability" between April 1830 and November 1831, and what was it based upon (some scripture? (unrecorded) revelation?)?