For your morning enjoyment, here's a delightful way to view the change, over time, of the US population, in terms of the relative size of various age groups.
It goes all the way from 1900 to 2060, and you can nicely observe phenomenons like the baby bust, the baby boom, the current retirement boom, and the future periods when the boomers will die off.
It was put together by the brilliant Calculated Risk.