The years of our life are seventy,A drop in a bucket. 70 years, 80 years... The numbers matter so little in the scheme of eternity. They are just a drop in a very large bucket.
or even by reason of strength eighty;
yet their span is but toil and trouble;
they are soon gone, and we fly away.
The point that the Psalmist is trying to make here doesn't have anything to do with statistics. He is not trying to give us the average life expectancy of a Jewish male living during this particular century. He is trying to show that we, when we are thinking small, when we are thinking human, seem to think that 80 is a big number.
I tend to think this way when I think about myself. I am 35, and I get excited about that. I can't believe I am that "old". Or when I think about how long I have been teaching, I am still a newbie, but I am in my 8th year. In my human ears, that sounds like a while. My oldest son is 9. He seems so old to me. It is weird sometimes.
Remember though, "...they are soon gone, and we fly away."
Train yourself to think eternally.