Before the modern era, life expectancy was no more than 35 years and as low as 20 years in some places. By 1900, life expectancy in the United States had increased to 47 years (which is the current life expectancy in Nigeria). By 2000, it was about 77 years in the United States and more than 80 years in several countries, including Australia..
Life expectancy has increased because of better sanitation and nutrition and through medical advances. If these continue to improve, presumably life expectancy will continue to increase. As people live longer, more advances will occur in their lifetime – and they will live longer still.
World population is predicted to stabilize at around 9 billion by about 2070. But to keep the population stable, the number of births must equal the number of deaths. If we keep increasing our lifespan, we must keep reducing the birth rate.
If we ever reach the stage at which the age of death is being advanced by more than one year per year (It’s now advancing at about one year every three years), the birthrate will have to be zero to keep the size of the population stable.