42 USC 1395o - Eligible individuals

Every individual who
(1) is entitled to hospital insurance benefits under part A of this subchapter, or
(2) has attained age 65 and is a resident of the United States, and is either
(A)  a citizen or
(B)  an alien lawfully admitted for permanent residence who has resided in the United States continuously during the 5 years immediately preceding the month in which he applies for enrollment under this part,

is eligible to enroll in the insurance program established by this part.