Mean as can be

 

Any statisticians among us?  I read in the New Yorker in an article about the history of cancer treatment ("Cancer World" by Steve Shapin) that the apparent increase of cancer is in part a result of increased longevity.  "At the beginning of the twentieth century,life expectancy at birth in America was 47.3 years... The median age at diagnosis for breast cancer in the United States is now sixty-one; for prostate cancer it is sixty seven; for colorectal cancer it's seventy."  

What does this median actually mean, and how does it relate to longevity?  What is the median?  Does it mean th fewer people are diagnosed before and after that point? That doesn't seem likely.