The reason for the twelves is that the numbers were incorrectly translated from cuneiform into Hebrew. Sumerians used 12 and 60 (5x12) in their numbering systems, thus modern clocks have 24 hours and 60 minutes, and circles have 360 degrees. If you translate the numbers back into cuneiform and then correctly translate them again, the ages are all similar to the ages people live today.

Source: Noah's Ark and the Ziusudra Epic: Sumerian Origins of the Flood Myth by Robert M. Best