I have been wondering what we can know, Biblically speaking, about the number of the days of our individual lives. Is it a predetermined number that we cannot alter by lifestyle or by choices we make (i.e. engaging in extreme or hazardous sports)? I have looked at Psalm 139, Job 14:5, Proverbs 3:1-2, Proverbs 9:11, Eccl. 7:15, etc. It might be reasonable to think that some choices we make (i.e. dietary, exercise, flossing, to name a few) could improve the quality of our life, but does anything under our control increase the number of days we are given?
In Matt. 6:27 Jesus said, “Who of you by worrying can add a single hour to His life?” From this and the verses you quoted, it would seem that our life spans are finite and known in advance. However, based on my knowledge of God’s character and His capabilities I don’t think this is the result of an arbitrary determination on His part. God, who knows the end from the beginning, knew before we were born what choices we would make and how these choices would influence our life span. He also knew what diseases we would contract, what accidents would happen, etc. This foreknowledge allowed Him to calculate the length of our life. Therefore none of the choices we make over the course of our life will add to or subtract from the number of our days because these choices have already been factored into the calculation.