I am no sure if there is a specific scripture that deals with it but I think our days are counted and our days could also be shortened by the choices of other people.
I recall reading the story of some early pioneers (A family). They all died at the same time and one of the early brethren said the reason for that is that the Lord wanted them all together for a special mission in the other side so maybe there IS a time we are set to die I guess.
Name: Thevestsun
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Comments: I don't know that this is exactly church doctrine but I have read a lot about how we each have chosen many of our lives circumstances and trials and even our families before we came here. With the help of Angels and the approval of Heavenly Father. But it is my belief that we have always had free agency and are in control of our own lives and that some of us have even chosen the circumstances of our death before coming here and that we often pick the circumstances that will allow the most growth for the ones we love and the best opportunity for us in the scheme of eternity to return to our father. Even from the beginning. But that Heavenly Father will always help us when we ask and he will intervene at times so that specific plans are not frustrated. Just my take.