From the manual:
This parameter determines whether like terrain squares clump together, or are widely scattered.
3 Billion Years: This option yields a young world, one in which terrain squares seem to occur in clusters.
4 Billion Years: This option yields a middle-aged world, one in which glaciation and plate tectonics have been acting to diversify terrain.
5 Billion Years: This option produces an old world, one in which the forces of nature and chaos have almost wholly randomized the terrain features.