What, historically, was the age of accountability for Latter-Day-Saints before D&C 68:27? In Doctrine and Covenants 68:27, the age of accountability is clearly defined as being eight years.

D&amp;C 68:27


  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?)?
It is clearly taught in the Book of Mormon that baptism is only required for those who are accountable and capable of committing sin. Moroni Chapter 8 contains a letter of Mormon to his son expounding on the theology of the subject. (Especially verses 5-26).
Mosiah 3:16-21 also contains teaching that "little children" are sinless and are redeemed by Christ. The passages mentioned in the Doctrine and Covenants (18:42 and 20:71) also mention "the years of accountability" but are not more specific. 

At this point in the history of the Church, (November 1831) there were no more than a few thousand members and whether someone was sufficiently accountable was most likely left to the judgement of the missionaries, parents, and the individuals involved. There were some young children among the newly baptized convert families, but only a few would have aged and matured enough for the question of when they should be be baptized to arise.

Joseph Smith taught (Genesis 17:11-12; Joseph Smith Translation) that the 8 year old age for accountability was connected to the 8 days of age for circumcision according to the law given to Abraham.