People live longer; diseases of old age are not cured yet and are considerably expensive and labour intensive. You need lots of people earning money and paying taxes to provide for that older generation. You also need lots of people to do the physical care of that population.
This works in some kind of flat tax system, which isn't really how it works in the US.
Fresh college graduates end up with significant education debt, and interest payments on student loans are deductible. For the rest the system of credits allows for zero net tax obligation as far as federal budget is concerned http://www.csmonitor.com/Business/Donald-Marron/2011/0728/Wh...
US economy is dependent on high income earners, which might or might not be younger people.
Fun sci-fi noodling would have criminals being sub-contracted into institutions that care for poverty stricken old people. The prison / elderly care industrial complex grows to a huge evil empire. (Throw in a bit about making and selling coffins and it's complete.)
People live longer; diseases of old age are not cured yet and are considerably expensive and labour intensive. You need lots of people earning money and paying taxes to provide for that older generation. You also need lots of people to do the physical care of that population.