Genesis 6:3 - Life Span Shortened

And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.

God's Limit on Mans Years by Guy Cramer points out that the 12o years is the limit God placed on man's life. After this declaration was made lifespans began to shorten until it was less than 120 years. Guy Cramer notes that:

some scholars suggest that this 120 years was a prophetic time line until the flood took place. However, we see God's decision was made in Genesis 6:3 after Noah turned 500 in Genesis 5:32. Noah was 600 years old when the flood waters were on the earth. Genesis 7:6 NKJV) Taking these passages we can determine that the flood took place within 100 years after God's decision. So the prophetic interpretation of the Genesis 6:3 is a wrong one.

I agree with Guy Cramer's interpretation. When God makes a declaration it doesn't have to happen immediately. Adam and Eve didn't die immediately after eating the forbidden fruit from off the tree of knowledge.