My understanding is that the doubling in life expectancy over the last 150-200 years was primarily due to the drastic reduction in infant mortality and elimination of most infectious diseases. If you only look at life expectancy from the point of reaching adulthood, the change hasn't been nearly as dramatic.
But if you eliminate life expectancy at birth, you'll notice that there hasn't been a drastic improvement. It's been discussed on HN previously, and I think adult life expectancy has only improved by less than 20 years over the last several hundred.
(Someone with sources feel free to correct me).