(AP) -- Most Americans have never known a world where man hasn't been to the moon. It used to be a given that people knew where they were when man first walked on the moon on July 20, 1969, watching the black-and-white images on television. But now most Americans don't know where they were because the majority of Americans hadn't been born yet.
The median age of Americans, as of last year, was 36.8, meaning more than half of U.S. residents are younger than 40, according to the U.S. Census Bureau. No figures have been calculated for this year yet.
Five years ago, when NASA celebrated the 35th anniversary of the moon landing, the median age of Americans was 36.1, so most residents were at least alive when Armstrong made his giant leap for mankind.
That changed sometime between July 2005 and July 2006, according to the Census Bureau.
Six current astronauts were not alive when Armstrong walked on the moon, including Christopher Cassidy, a crew member of the ongoing mission on the space shuttle Endeavour.
"It tells us time passes," Smithsonian Institution space curator Roger Launius said. "You've got under 40 people all over the place."
Time also passes for the Apollo 11 astronauts. Neil Armstrong and Michael Collins were 38 when Apollo 11 landed on the moon; Buzz Aldrin was a few months older at 39.
So all three Apollo 11 astronauts have now lived most of their lives since what is considered the crowning achievement of a lifetime.
It has been a challenge to "carry on with the rest of your life," said Aldrin, who battled depression and alcoholism. He said there is "this uneasiness and this uncertainty as to what I really ought to be doing."
©2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Explore further: Researchers find giant convection cells on the Sun