[timbit42]

Genesis doesn't say people's lives are limited to 120 years. What it says is that the flood would occur in 120 years time.

That said the numbers in Genesis were incorrectly translated from cuneiform into Hebrew. If you translate them back into cuneiform and then forward again correctly, you get smaller numbers that match up with the ages people have children or die today.

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