After we sinned in the Garden of Eden, we immediately began to degenerate and die. Plus, our life expectancy steadily decreased from Adam (who died at just under 1,000 years old), all the way down to 120 years I think you are misinterpreting the bible. Several are listed as living longer than Adam so your statement about 'immediately began to degenerate does not correlate. Also, if Adam began to immediately degenerate how did he manage to live to 930?
You are correct, there was a bit of a wobble just after Adam, but after that there was a steady decline. And the fact that Adam died at all means he degenerated, as per the curse in Genesis.
