You're ignoring the number of contributors who don't live long enough to collect their pension. My wife's sister at 54, my business partner at 64, my colleague at 63, my dentist at 55, I could go on and on... The life expectancies you quote are for different sets of people. The contributions that they made when they started work at 16 (15 in some cases)weere sufficient at the time.
The confusion now is that there are different classes of pensioners, those who contributed all their lives and have never lived off the state, those who have never contributed anything yet still get a pension and a whole range of people in between.