As mentioned above, without explaining your magic system in more depth, it is very difficult (some might say impossible) to answer your question. However, might I suggest a more quantile view of a person's life force.
Regardless of how old you are or how much life you have left to live, your life has the same value in the eyes of magic. It is not that you are somehow taking your unused years away from Fate and using them to fuel a spell, for Fate has already decided that you will die in this moment by taking your own life. Nay, it is the magic of crossing the barrier between life and death that you are manipulating.
Alternately, for a more physics-based approach, you can think about a person as a fuel source (I mean, if you set us on fire will we not burn?). We constantly take in energy and return it to the environment, but the total amount of energy in our system is far greater than the amount of energy we are able to tap into before dying of exhaustion. In this way, the amount of energy produced from hypothetically accessing all the potential energy stored in our system at once has nothing to do with how long we have left to live; it is instead measured by our physical mass.