Adam is the one who started all of this. When Adam sinned, sin entered the world and affected the entire human race. With the sin, death entered the world and it too affected the entire human race. Therefore, through the one man sin entered into the world and death caught a ride, both affecting the entire human race. Adam’s sin spread death throughout all the world, so everything began to grow old and die (for sin entered into the world and death followed sin into the world).
We know that it was Adam’s sin that caused this because according to the scriptures, people were sinning from the time of Adam until Moses and God did not judge them guilty of death for breaking his laws, because he had not yet given his laws to them, nor had he told them what he wanted them to do. We also know from the scriptures that when their bodies died it was not for their own sins since they themselves had never disobeyed God’s special law against eating the forbidden fruit, as Adam had.