[W]hat really matters in a pay-as-you-go system is the dependency ratio, i.e., the ratio of workers paying into the Social Security system to the number of Social Security beneficiaries. The number of over-65s is a lot higher today than it was at Social Security’s inception. This would have been challenging enough on its own, but a series of Carter-era measures increased the rate at which Social Security benefits grow. It is also worth noting that over-65s were the most impoverished age group when Social Security was established, and that distinction now belongs to under-18s.
Andrew Sprung is in related territory when he mentions that "many more baby boomers will reach age 65 (or 67, or 69) than did their predecessors," which will effect the dependency ratio. Ryan Avent, meanwhile, is willing to consider raising the retirement age, but, like Ezra Klein, he worries about the consequences.