My take:

Aging populations living longer, rising costs of living, lack of education among the poor, having more children to support relative to the mean (also starting at an earlier age), poor ability to save or an inability in some cases due to again lack of education and a rather dismal minimum wage, growing inequality (confounding factor), inability to invest due to no savings, higher unemployment rates among the poor, and so on...

Also, an analysis of change in the population pyramid relative to GDP over time in the US would be pertinent to the discussion. Not sure if anyone is up for a 3-D analysis of that sort, neither can I find anything on the web in regards to such an analysis.