In truth, we already do ration health care for the elderly and for everyone else in our society. We do not call it rationing because we do not have the government telling us which procedures we can and cannot have done.
However, we do ration health care. We only provide most kinds of health care to those who can afford it. This tends to mean that we provide it to those who have health insurance. We also ration it by providing only such health care as health insurance companies will cover. So, we actually do ration health care and we should be honest about the fact that we do.
That said, I think your question is asking if we should deny care to the elderly based on the idea that they will soon die anyway and do not need the care. This is, of course, something that we cannot really do. We cannot tell the elderly that they do not deserve care because they do not have as long to live. However, elderly people, their doctors, and their insurers probably should put some thought into the costs and benefits of the procedures that they order. They should not, for example, order expensive diagnostics to test for cancers that are not likely to kill someone (or significantly reduce their quality of life) before the person dies of natural causes. I think that we have to be somewhat more realistic and not have the mindset that more procedures are better in all cases.
So, I do not think that we should "ration" health care. However, I do think we should be much more intelligent about what care we provide and consume.