Fewer young people paying for the old people.

The basis of what the real problem is with Social Security -- in Newsweek:

Which brings us to Social Security's financial "crisis." The issue isn't that Social Security is spending too much or that we're living too long. It's that we're not having enough children (or letting in enough immigrants). As Stephen C. Goss, the system's chief actuary, has written, Social Security projects an imbalance "because birth rates dropped from three to two children per woman." That means there are fewer young people paying for the old people. "Importantly," Goss continues, "this shortfall is basically stable after 2035." In other words, we only have to fix Social Security once.