Calculated Risk (scroll down) provides a great set of charts showing the age structure of the US population from 1920 through 2005. The story has several components--the "baby bust" of the Depression years (which emerges clearly in the charts for 1930 and 1940), the "baby boom" that followed World War II and is still visisble in the charts, and the overall flattening of the age distribution (look particularly at the chart for 2005--the 5-year age groups (0-4, 5-9, 10-14, etc.) from 0-4 through 50-54 all have right around 7% of the total US population. Only at that point do the sizes of the older age groups begin to shrink. And even those older age groups are MUCH larger than earlier--slightly more than 9% of the population now appears to be 70 or older, compared with 8.4% as recently as 1980, and 5.8% in 1960. It's not impossible that the percentage of the population that is age 70+ will rise to about 13% by 2050. It's not clear what the implications of that are, but stay tuned.