Why isn't Medicare available to all Americans?

In the middle of the health care insurance debate, I thought I'd turn the single payer issue around. I favor a single payer system, basically Medicare for everyone. Then I wondered: why isn't Medicare available to all Americans?

Or, to put it another way: how can it possibly be legal to limit it by age? Why isn't that age discrimination?

Obviously, I'm not a lawyer or I would not ask such silly questions. If it were so clear a case of age discrimination, why wouldn't a group of young lawyers have already filed a case about this?

Here is the eligibility: "people age 65 or older, some disabled people under age 65, and people of all ages with End-Stage Renal Disease (permanent kidney failure treated with dialysis or a transplant)."

http://www.medicare.gov/MedicareEligibility/home.asp?version=default&browser=Safari|2|WinXP&language=English

Still, it makes me wonder. Maybe we can save the president and Congress the agony of making decisions about health care insurance. Just have young people sue for coverage and see what happens. Maybe the solution to health care insurance has been there all along.