The issue could still be argued in terms of the ethics of withholding care, but it is not discriminatory. Age Discrimination vs. Race Discrimination

In principle, age discrimination is like race discrimination in that it results in different rights or treatment of certain people because of a shared characteristic and also because that characteristic is not something that is the result of choice. Just as nobody can control his or her ethnicity or skin color, nobody can avoid aging or deteriorating in health as a function of advancing age. However, age discrimination and race discrimination are much more different than they are similar, mainly because everybody ages and because there are objective rationale for certain kinds of different treatment of people based on chronological age that do not apply to racial discrimination. For example, race does not determine how physically capable a person is of performing a set of tasks or how long he or she may continue working for an employer, or how much he or she is likely to miss work because of health problems or how much the employer may have to pay out in healthcare costs over that employee's career with the organization.

Feasible Possible Alternatives

Instead of asking about or framing the issue in terms of the chronological age of patients, it might make more sense to focus on the underlying reasons that age is sometimes a reason to question...

For example, there is nothing age-discriminatory of excluding cancer patients from organ transplants or from heart bypass surgery; and there is nothing age-discriminatory about excluding Alzheimer patients from major surgical intervention intended to prolong life at public expense. Certainly, all of those medical conditions are associated with advanced age in a correlational way; but the exclusion would not consider age per se. It would reflect the objective purpose of not using public funds to prolong life beyond the point where doing so could be futile from the cost-benefit or the relative quality of life. Under that approach, one-80-year-old patient might be denied coverage for a heart bypass because he has advanced cancer or Alzheimer's; but another patient of the same age without those issues and with a prognosis of living another five years might qualify for the same coverage. Admittedly, that introduces ethical issues in connection with relative material wealth and a two-tiered healthcare system based on wealth, but it goes a long way toward solving any problems over age discrimination.
Reference

Yates L.B., Djousse L., Kurth T., Buring J.E., and Gaziano J.M. "Exceptional

longevity in men: modifiable factors associated with survival and function to age

90 years." Archives of Internal Medicine, 168(3) 2008: 284 -- 290.