The really big story of the past century

Go, grandma!
Look to your left. Look to your right. Without advances in agriculture and medicine, one of you would not be alive today.

And yet you are. America's population of centenarians - already the largest in the world - has roughly doubled in the past 20 years to around 72,000 and is projected to at least double again by 2020, perhaps even increase seven-fold, according to the Census