Medicare Eligibility and Enrollment

Program Administration and the Structure of Medicare

Medicare is a federal health insurance program established by the Social Security Amendments of 1965, which added Title XVIII to the Social Security Act. It is administered by the Centers for Medicare and Medicaid Services, which is an agency within the United States Department of Health and Human Services. While CMS oversees policy, other entities manage consumer interactions. The Social Security Administration handles most Medicare eligibility determinations and enrollment, whereas the Railroad Retirement Board handles enrollment for eligible railroad beneficiaries. Claims under Original Medicare are processed by private insurance entities known as Medicare Administrative Contractors.

Medicare is organized into distinct parts to address specific healthcare needs. Original Medicare consists of two parts: Part A for hospital insurance and Part B for medical insurance. Beyond Original Medicare, Medicare Part C refers to the Medicare Advantage program, which allows private health plans to deliver Medicare benefits. Medicare Part D provides outpatient prescription drug coverage administered through private insurance plans.

Eligibility Criteria and Part A and Part B Costs

At age 65, an individual may qualify for Medicare if they are a United States citizen or a lawful permanent resident who has lived continuously in the United States for at least five years. A person generally qualifies for premium-free Part A if they or a qualifying spouse have at least 40 quarters of Medicare-covered employment. According to CMS, about 99% of Medicare beneficiaries pay no Part A premium in 2026; a person may qualify through their own covered work or a qualifying spouse's work record. People who lack enough covered work may be able to buy Part A if they meet Medicare eligibility requirements and enroll in Part B. In 2026, people who buy into Medicare Part A with 30 to 39 quarters of coverage pay a reduced premium of $311 a month, while those with fewer than 30 quarters pay the full premium of $565 a month.

For Part A covered inpatient care in 2026, beneficiaries pay an inpatient hospital deductible of $1,736 per benefit period, which covers the beneficiary's share of costs for the first 60 days of inpatient care. If a hospital stay extends longer within that benefit period, the beneficiary pays daily coinsurance. In 2026, Part A hospital coinsurance is $434 per day for days 61 through 90, and $868 per day for lifetime reserve days. For extended care in a skilled nursing facility, the daily coinsurance for days 21 through 100 in a benefit period is $217 in 2026.

Part B covers outpatient medical services and requires a monthly premium. In 2026, the standard Medicare Part B monthly premium is $202.90, and the annual Part B deductible is $283. Part B premiums are higher for beneficiaries with higher incomes through income-related monthly adjustment amounts, which affect roughly 8% of people with Part B in 2026. However, Medicaid or another assistance program may pay the Part B premium for an eligible beneficiary. When a beneficiary receives Social Security benefits, their Part B premiums are generally deducted directly from those payments, unless another program pays them. If you do not receive Social Security or Railroad Retirement benefits and have no third-party assistance, Medicare generally bills you for Part B premiums, often quarterly.

People under age 65 can also become eligible for Medicare under certain circumstances. Most qualify after 24 months of entitlement to Social Security disability benefits, with Medicare coverage beginning automatically in the 25th month. Different rules apply to specific conditions. People with ALS who qualify for Social Security disability benefits become eligible for Medicare in their first month of disability benefit entitlement, without the usual 24-month waiting period. People with end-stage renal disease who need regular dialysis or a kidney transplant may qualify if they meet applicable work or benefit eligibility requirements. For a person qualifying due to end-stage renal disease who receives dialysis, coverage generally begins on the first day of the fourth month of dialysis, though it can begin earlier in certain situations, such as qualifying home dialysis training.

Enrollment Periods and Late Penalties

How and when you enroll in Medicare depends on your current retirement benefit status. People already receiving Social Security or Railroad Retirement benefits are generally enrolled automatically in Medicare Parts A and B when they become eligible at age 65. If you are not automatically enrolled, you generally must sign up for Medicare through the Social Security Administration, or through the Railroad Retirement Board if you receive Railroad Retirement benefits.

The age-65 Initial Enrollment Period generally lasts seven months. This period spans the three months before your 65th birthday month, your birthday month, and the three months after it. Enrolling early prevents coverage delays: if you enroll in Part B during the first three months of your age-65 Initial Enrollment Period, coverage generally starts on the first day of your 65th birthday month, or the first day of the preceding month if your birthday is on the first day of a month.

Eligible people who miss their initial enrollment window can sign up during the General Enrollment Period, which runs from January 1 through March 31 each year. Coverage selected during this window takes effect on the first day of the month following the month of enrollment. However, waiting can lead to penalties. If you must buy Part A and enroll late, the Part A premium generally increases by 10% for twice the number of years you could have enrolled but did not. Without a qualifying exception, the Part B late enrollment penalty is 10% of the standard Part B premium for each full 12-month period you could have enrolled but did not, and this penalty generally lasts as long as you have Part B.

You can avoid the Part B late enrollment penalty if you were covered by an employer group health plan based on active employment when first eligible. If you have qualifying employer group health coverage based on your or your spouse's current employment, you can generally enroll in Part B without penalty while that coverage continues or during a Special Enrollment Period. This Special Enrollment Period lasts for eight full months starting the month after employment ends or the group health coverage ends, whichever occurs first. If you delayed Part B because of qualifying employer group health coverage, signing up during this Special Enrollment Period helps avoid a coverage gap and a penalty. Be careful with other coverage types: COBRA continuation coverage and retiree health plans do not count as current employment coverage for Medicare Special Enrollment Period purposes.

Comparing Original Medicare and Medicare Advantage

When eligible for Medicare, beneficiaries can receive their benefits through Original Medicare or through Medicare Part C, known as Medicare Advantage. With Original Medicare, you can visit any doctor or hospital in the country that accepts Medicare, and a provider does not have to accept assignment to see Medicare patients. Assignment means the healthcare provider agrees to accept the Medicare-approved amount as full payment for covered services. Original Medicare does not establish an annual maximum out-of-pocket limit, meaning beneficiaries remain responsible for deductibles, copayments, and coinsurance throughout the year. To mitigate this exposure, individuals with Original Medicare and Part B may buy a private Medigap policy to help pay certain deductibles, copayments, and coinsurance, subject to eligibility and enrollment rules.

Medicare Advantage provides an alternative managed care pathway. To enroll in a private Medicare Advantage plan, an individual must be enrolled in both Medicare Part A and Medicare Part B. These plans, which include HMOs and PPOs, must generally cover services covered under Original Medicare Parts A and B; Original Medicare continues to cover hospice care and certain costs of clinical research studies for Medicare Advantage enrollees. Unlike Original Medicare, Medicare Advantage plans establish a mandatory annual limit on out-of-pocket spending for covered Part A and Part B medical services. Plans frequently include integrated Part D prescription drug coverage and additional benefits such as routine vision or dental care. However, enrollees usually must use a specific network of doctors and hospitals, giving them narrower provider choice than Original Medicare.

Medicare products cannot always be combined. A Medigap policy cannot pay Medicare Advantage cost sharing, and an insurer generally cannot sell you a Medigap policy while you are enrolled in Medicare Advantage unless you are switching back to Original Medicare. Furthermore, consumers should understand what basic Medicare excludes: Original Medicare does not cover most routine dental care, routine vision exams, hearing aids, or long-term custodial nursing home care.