The example of Rita Montalchni is incredibly interesting (https://en.wikipedia.org/wiki/Rita_Levi-Montalcini). She administered a nerve growth factor (NGF) as eye drops and lived for 101 years while her twin sister died when she was 91. (Bearing in mind the average life duration difference of twins is six years, we can conclude that she gained about four years.)

That's not how it works. You can't do calculation on this on a individual basis.