U.S. Population Changes Over Time...

This is a fascinating animated graph by Bill McBride at Calculated Risk.  Amongst other things, it makes obvious the reasons for the current and future insolvency of the Social Security system.  Way back when, before FDR, there were vastly more people of working age than of retirement age.  Today, not so much.  In the future, even less.

Of course there are many other repercussions of the demographic changes as well.  For instance, think what it means for healthcare costs, or number of doctors needed per capita, or economic growth, or any of dozens of other topics.

Study this for a while and you'll understand a lot about the dynamics that are affecting this country.  It might also make you rethink opposition to immigration (the vast majority of immigrants are under 30 years old)...