As the earth falls in space it continues to degrade in the quality of the matter it is formed of as it becomes more corrupt as the fall moves towards it's final corrupted mortality state from which it is ultimately redeemed:
It is possible that, as the above diagram illustrates, as the matter becomes more corrupted it is different from matter that Adam's body was comprised of. Thus we see the declining ages as a natural result of the process of a fall from states of perfected matter to states of increasingly corrupted matter until we get to where we are now with life spans around 80 years or so.(Alma 41:2-6.)
2 I say unto thee, my son, that the plan of restoration is requisite with the justice of God; for it is requisite that all things should be restored to their proper order. Behold, it is requisite and just, according to the power and resurrection of Christ, that the soul of man should be restored to its body, and that every apart of the body should be restored to itself.
3 And it is requisite with the justice of God that men should be judged according to their works; and if their works were good in this life, and the desires of their hearts were good, that they should also, at the last day, be restored unto that which is good.
4 And if their works are evil they shall be restored unto them for evil. Therefore, all things shall be restored to their proper order, every thing to its natural frame—mortality raised to immortality, corruption to incorruption—raised to endless happiness to inherit the kingdom of God, or to endless misery to inherit the kingdom of the devil, the one on one hand, the other on the other—