This is, for example, Greg Mankiw: 'a looming fiscal crisis, which threatens to unfold as baby boomers retire and start collecting Social Security and Medicare' (and, unrelated to the topic, here's an entertaining quarrel)
This graph from the Congressional Budget Office flies to the face of this mantra:
A presentation is here, and here's interesting commentary:
Although many observers portray aging as the dominant cause of future growth in federal spending on Medicare and Medicaid, most of the increase that CBO projects reflects rising costs per beneficiary rather than rising numbers of beneficiaries. The effect of population aging is smaller.
That said, note that this (i.e. Medicare and Medicaid commanding 20% of GDP), simply can't happen, and the only thing that can give is the 'excess cost'.
What this graph says one thing only: we should expect the growth rate of medical knowledge to slow down in the years to come.