Genesis 1:1 - 31
ESV - 1 In the beginning, God created the heavens and the earth. 2 The earth was without form and void, and darkness was over the face of the deep. And the Spirit of God was hovering over the face of the waters.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
In Genesis 6:3 When the Lord said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.”(NIV) He was not establishing a natural age for each person from that time forward or even beginning after the Flood. Genesis 6:5-13 gives the reason for this statement. God had seen man's wickedness on Earth and He vowed to destroy all mankind and the Earth. The 120 year period was to be the amount of time before the Flood that mankind was given a chance to Repent, which they never did.
Well, I think that because Adam and Eve were in the garden and they had two special trees. The tree of life which grants immortality and the tree of the knowledge of good and evil. Our Lord only forbade them from eating from the second tree. There were no restrictions against the tree of life, they could eat it all they wanted.
I think the long lifespans were a residual effect from Adam and Eve eating from the tree of life. If you'll notice these people had gradually shorter lifespans, so it can be reasonable to conclude that the immortality was gradually being bred out of them.
While Adam and Eve could have ate of the tree, scripture never says that they did. God banned them from Eden and the tree of life after they sinned.
It seems more likely that the long ages were a product of un-deteriorated genetics, coupled perhaps with the greater protection from the radiation of the sun which the pre-flood world probably had.
I often felt as though the earth was different back then as it is now. A lot changed during and after the Flood. The earth itself, spun faster. Thus making the days shorter back in biblical times. Once the oceans were filled, and the land masses were moved, the earth's rotation shifted, and slowed down dramatically, thus making the days longer. I have nothing to back my theory up, just something I just always figured happened.