This ignores the (I think) very strong possibility that the old people are preferentially selected by the system because, thanks to Medicare, they can _afford_ the ICU. Many people aged less than 65 cannot. Consider the idea that if we had something like "Medicare for all", the population of the ICU would better reflect normal demographics.
That said, as a beneficiary of Medicare I can only be grateful. I had several days in a top-quality hospital and a procedure by a top-quality surgeon, and after all the EOBs had come in, I ended paying out of pocket... nothing at all.