In the first edition of Civilization, the length of each turn varies. It starts at 20 year before 1AD and is one year after 1900. What is the full table of turn length?
In the original Civilization, the years per turn breaks down in the following way:
I believe that turns count for fewer years as the game progresses to simulate the accelerating technological progress of civilization - it takes us less real-world time in the modern era to make larger leaps forward than it did in previous historical eras.
The game ends in a different year depending on the difficulty setting:
Essentially, the easier the difficulty level, the more turns you have to finish the game.