Imagine you have a metering rule made of paper that is one meter long, like the ones you find in Ikea. Cut the metering rule according to your current age. For me this means, I turn 40 years old this year, so I cut off the first 40 centimeters. Now, what is the life expectancy in your country? In Germany, it is currently 83.4 years for women. Therefore, I cut off the remainder of the metering rule from 83.4 to 1 meter.
What is left after this procedure? The lifetime that I probably have left.