I found an interesting paper on how age pyramids can be used to explore the past. This is a problem that I have come around to over and over before, as it is so difficult to estimate even without modern census technology what the compositions of past populations might have been. How could the Angle-Saxons of Beowulf's time have sustained the rates of raiding and attack, given their birth rates and the lethalness of their battle wounds in the age before proper infection control? Clearly the age pyramids would provide clues ... but how to get them? Working backward from similar current age pyramids seems to be about the best way to do that.
Of similar interest is this post with more links to other age pyramids.