Green card holders who do not qualify for Medicare will qualify for Obamacare and any relevant federal subsidy. They may also qualify for Medicaid, depending upon their location.
One needs to have worked a total of 40 quarters (10 years) in the United States to qualify for Medicare.
U.S. citizens and lawful permanent residents at least age 65, who have been here five years, may buy into Medicare if they don’t have the work history. Individuals who qualify to buy-in to Medicare can choose to participate in Medicare or buy Obamacare insurance.