On this issue I would recommend reading Genesis Study by Henry Morris, the True Believer in Jesus who was called by Lord in February this year.
 
Shortly speaking, the human life time has changed many ways.
 
After the sin came into this world, we have the limited life time.
Before the flood of Noah, the water-layer in the sky prevented the UV's from penetrating very much.
After the collapse of the layer of the water mist in the sky, UV was increased enormously on this earth, which deteriorated the human genome very much.
Even after the Noah's era, the human life time was still long, because it took some more time after generations, until the human life time was shortened remarkably.
This required the people to avoid the marriage between the relatives as the genome deterioration caused the problems.
 
After the Flood, the life time was reduced continuously, related to UV's and Genome deterioration.
 
On the other hand this leaves us the hope that our life can be extended quite a lot when we obey God, even before we reach the Kingdom of God.