The scope of Medicare is established by statute which must be amended with a statute approved by Congress and signed by the President, or approved after being vetoed by the President with a veto override, to be changed. The age limitation is set forth at 42 U.S.C. 1395c:
The insurance program for which entitlement is established by sections
226 and 226A provides basic protection against the costs of hospital,
related post-hospital, home health services, and hospice care in
accordance with this part for (1) individuals who are age 65 or over
and are eligible for retirement benefits under title II of this Act
(or would be eligible for such benefits if certain government
employment were covered employment under such title) or under the
railroad retirement system, (2) individuals under age 65 who have been
entitled for not less than 24 months to benefits under title II of
this Act (or would have been so entitled to such benefits if certain
government employment were covered employment under such title) or
under the railroad retirement system on the basis of a disability, and
(3) certain individuals who do not meet the conditions specified in
either clause (1) or (2) but who are medically determined to have end
stage renal disease.
Also, money doesn't grow on trees. The only way to provide that greatly expanded single payer health care benefit would be to raise hundreds of millions of dollars or more of new taxes to pay for it. Any new tax would have to be proposed with a bill initiated in the House of Representatives, approved by the Senate, and then passed by the President (or approved over his veto with a veto override).
Likewise, the increased appropriation of funds for the benefits would have to be approved by Congress, even though it is an entitlement, because current legislation doesn't authorize spending for that purpose.