I'm not sure why you feel that life expectancy at age 20 is the same statistic as life expectancy at age 50, but to answer your question I chose age 20 because

I'm not sure why you feel that life expectancy at age 20 is the same statistic as life expectancy at age 50, but to answer your question I chose age 20 because many people seem to think that the only major improvement has been in childhood mortality. But clearly there has also been significant improvement in adult mortality - at all ages.