No one is competent to determine the length of anyone's life. Why then is there mention of a hundred years span for man in the scriptures?
This is not mere fancy of the authors of the scriptures. It is the truth. Every man should live for a hundred years. What is the reason for premature death?
The wicked tendencies in man, such as arrogance, envy, bad thoughts and misdeeds, go on shearing the life of man into pieces. His bad qualities shorten the life of man. When a man's life is filled with good thoughts and good actions, he can live for a hundred years. The secret of the longevity enjoyed by the ancient sages is precisely this. Man today wastes his life in bad thoughts and bad actions and ends his life in a bad way.
(Gems of Wisdom, Spiritual Level, pg 263)


