How Has Human Life Expectancy Changed?

I read that while human life expectancy has increased, human lifespan remains the same. If so, how can scientists account for the rising age of the oldest living humans since 1800, from about 100 years old to over 120 most recently?

—B.J., St. Louis, Mo.

The outer limits of the human lifespan are genetic, but increased life expectancy (the oldest age in that lifespan that we’re likely to reach) is due to better nutrition, modern medicine and other environmental (non-inborn) factors. If the age of the oldest living humans has risen, it’s because some genetic groups may have slightly longer lifespans than others, and they are benefiting from those same factors.