What Are the Eligibility Requirements for Medicare Part B?

Medicare Part B is available to most people aged 65 and older who are U.S. citizens or lawful permanent residents, and to certain people under 65 with qualifying disabilities or specific medical conditions. Unlike Part A (hospital insurance), which most people get automatically and premium-free if they or a spouse paid Medicare taxes for at least ten years, Part B (medical insurance) requires active enrollment and a monthly premium. The eligibility rules are straightforward for most people, but the details around timing, special circumstances, and costs trip up a surprising number of enrollees.

The Standard Age Requirement

The most common path into Medicare Part B is turning 65. Once you reach that age, you become eligible regardless of whether you are still working, already retired, or have other health coverage. You do not need to be receiving Social Security benefits to qualify, though your enrollment process is simpler if you are. People already collecting Social Security or Railroad Retirement Board benefits at least four months before their 65th birthday are typically enrolled in both Part A and Part B automatically. Everyone else needs to sign up on their own.

There is no upper age limit. Whether you are 66 or 96, you remain eligible for Part B as long as you continue to meet citizenship or residency requirements and pay the monthly premium. And there is no health screening or pre-existing condition exclusion. Medicare Part B does not ask about your medical history before enrolling you, which distinguishes it from the individual insurance market that existed before the Affordable Care Act’s protections.

Citizenship and Residency Rules

You must be either a U.S. citizen or a lawful permanent resident (green card holder) who has lived continuously in the United States for at least five years immediately before enrolling. This five-year residency rule applies specifically to lawful permanent residents; U.S. citizens qualify at 65 regardless of how long they have lived in the country, as long as they meet the other criteria. People living abroad can keep Part B if they were enrolled before leaving, but the practical value diminishes because Part B generally does not cover health care received outside the United States, with very limited exceptions.

Qualifying Under 65 With a Disability

Age is not the only gateway. People under 65 can qualify for Medicare Part B if they have received Social Security Disability Insurance (SSDI) benefits for 24 months. The 24-month clock starts from the first month you are entitled to SSDI, not the first month you receive a check (there is typically a five-month waiting period for SSDI itself, so the total wait from disability onset to Medicare eligibility can be roughly 29 months). Once those 24 months of SSDI entitlement pass, you are automatically enrolled in Part A and Part B.

This waiting period is one of the more frustrating aspects of the system for people with serious disabilities. Someone diagnosed with a condition that prevents them from working may spend over two years without Medicare coverage, relying on COBRA, a spouse’s plan, Marketplace insurance, or Medicaid if they qualify. Legislation to shorten or eliminate the waiting period has been introduced in Congress multiple times but has not passed as of this writing.

End-Stage Renal Disease and ALS

Two medical conditions bypass the standard 24-month disability waiting period entirely. People diagnosed with end-stage renal disease (ESRD), meaning permanent kidney failure requiring dialysis or a kidney transplant, can qualify for Medicare regardless of age. Eligibility generally begins the first month of dialysis treatment (or in some cases the fourth month, depending on the type of dialysis and where it is performed) or the month a kidney transplant occurs. You must also meet other basic requirements: either you or your spouse must have worked long enough to be insured under Social Security, or you must already be receiving Social Security or Railroad Retirement benefits.

Amyotrophic lateral sclerosis (ALS, also called Lou Gehrig’s disease) is the other exception. People diagnosed with ALS become eligible for Medicare the same month their SSDI entitlement begins, with no waiting period at all. This change was enacted by Congress in 2000 because the 24-month wait was particularly devastating for a rapidly progressive disease. ALS remains the only condition with this immediate-eligibility provision.

For people with ESRD, the enrollment landscape has shifted in recent years. Before 2021, individuals newly diagnosed with ESRD were generally limited to traditional Medicare (also called Original Medicare) and could not newly enroll in Medicare Advantage plans except under narrow circumstances, such as already being in a Medicare Advantage plan when kidney failure developed. A provision of the 21st Century Cures Act changed that, opening Medicare Advantage enrollment to people with ESRD starting January 1, 2021.¹ This gave ESRD patients the same plan choices as other Medicare beneficiaries, though the financial nuances differ. For instance, people with ESRD who still have private employer insurance when they first qualify for Medicare are subject to a coordination period under the Medicare Secondary Payer Act, during which the private insurer remains the primary payer for the first 30 months of care.

What Part B Actually Covers

Understanding eligibility is easier when you know what you are signing up for. Part B covers medically necessary services and preventive care performed on an outpatient basis. That includes doctor visits, lab tests, diagnostic imaging, mental health services, durable medical equipment like wheelchairs and oxygen equipment, outpatient surgeries, and certain home health services. It also covers many preventive screenings at no cost to you, including annual wellness visits, flu shots, mammograms, colonoscopies, and cardiovascular screening blood tests.

Part B does not cover everything. Long-term nursing care, most dental work, routine eye exams for glasses, hearing aids, and most prescription drugs fall outside its scope. Prescription drug coverage comes through Part D, a separate program with its own enrollment rules and premiums. Many people pair Part B with either a Medicare Advantage plan (Part C) or a Medigap supplemental policy to fill these gaps.

Enrollment Periods and When You Can Sign Up

Eligibility does not mean you can enroll whenever you want. Medicare uses defined enrollment windows, and missing them can have real financial consequences.

Initial Enrollment Period (IEP): A seven-month window surrounding your 65th birthday. It starts three months before the month you turn 65, includes your birthday month, and extends three months after. If you are qualifying through disability, the IEP works similarly around your 25th month of SSDI entitlement. Signing up during the first three months of this window gives you the earliest possible coverage start date.
General Enrollment Period (GEP): Runs from January 1 through March 31 each year, with coverage beginning July 1. This is the fallback for people who missed their IEP and do not qualify for a Special Enrollment Period. Enrolling during the GEP usually triggers a late enrollment penalty.
Special Enrollment Period (SEP): Available if you delayed Part B because you had coverage through a current employer’s group health plan (your own employer or your spouse’s). You can enroll any time while you are still covered by that employer plan, or within eight months of the employment or coverage ending, whichever comes first. This SEP exists specifically so that people who have employer coverage do not get penalized for waiting. It does not apply to COBRA or retiree health plans, a distinction that catches many people off guard.

The Special Enrollment Period is the one that matters most for people still working past 65. If you have creditable employer coverage, you do not need to rush into Part B at 65. But once that employer coverage ends, the eight-month clock starts ticking, and there is no grace period beyond it.

The Late Enrollment Penalty

If you do not sign up for Part B when you are first eligible and you do not have qualifying employer coverage that would give you a Special Enrollment Period, you will pay a permanent penalty on your Part B premium. The penalty is 10 percent of the standard premium for each full 12-month period you could have had Part B but did not. And it does not go away. You pay the higher premium for as long as you have Part B.

As a practical example, if you went three full years without Part B and without qualifying employer coverage, your monthly premium would be 30 percent higher than the standard rate, permanently. For someone on a fixed retirement income, that adds up quickly over the years. This penalty is one of the strongest reasons to understand the enrollment windows and act within them, even if you feel healthy and are tempted to skip coverage.

What Part B Costs

Part B is not free for anyone. The standard monthly premium is set annually by the Centers for Medicare & Medicaid Services and is typically deducted directly from your Social Security check. Most enrollees pay the standard premium, but higher-income individuals pay more through an Income-Related Monthly Adjustment Amount (IRMAA). This surcharge kicks in at specific income thresholds based on your modified adjusted gross income from two years prior. People at the highest income levels can pay more than three times the standard premium.

Beyond the premium, Part B has an annual deductible and coinsurance. After meeting the deductible, you typically pay 20 percent of the Medicare-approved amount for most services, with no annual out-of-pocket maximum under Original Medicare. That unlimited cost-sharing exposure is one of the main reasons people buy Medigap policies or choose Medicare Advantage plans, which are required to cap your annual out-of-pocket spending.

Financial Help for People With Low Incomes

If the Part B premium is a hardship, several programs can help. Medicaid covers Medicare premiums and out-of-pocket expenses for low-income older adults, and this population represents a growing segment of Medicare enrollment.² People who qualify for both Medicare and Medicaid, known as dual-eligible beneficiaries, can have their Part B premium, deductible, and coinsurance paid by their state Medicaid program.

Even if you do not qualify for full Medicaid, you may qualify for a Medicare Savings Program. These state-administered programs come in tiers based on income and asset levels:

Qualified Medicare Beneficiary (QMB): Covers Part B premiums, deductibles, coinsurance, and copayments.
Specified Low-Income Medicare Beneficiary (SLMB): Covers Part B premiums only.
Qualifying Individual (QI): Also covers Part B premiums, at a slightly higher income threshold than SLMB.
Qualified Disabled and Working Individuals (QDWI): Covers Part A premiums for certain working disabled individuals.

Income limits vary by state, and many people who qualify for these programs never apply because they do not know they exist. Your State Health Insurance Assistance Program (SHIP) can help you determine whether you qualify and walk you through the application.

Common Situations That Cause Confusion

Several scenarios trip people up repeatedly. If you are covered under a spouse’s employer plan and that spouse retires, your Special Enrollment Period begins when the employer coverage ends, not when your spouse files for Medicare. People sometimes assume the two events are the same, but they are not always simultaneous.

Veterans who receive care through the VA health system still need to think carefully about Part B. VA coverage is not considered creditable coverage for purposes of avoiding the late enrollment penalty. If you rely solely on VA care and later decide you want to see doctors outside the VA system, you could face a permanent premium surcharge. Many veterans enroll in Part B as a safety net even if they primarily use VA facilities.

COBRA coverage also does not count. If you leave a job at 64, elect COBRA continuation coverage, turn 65 while on COBRA, and assume your COBRA counts as employer coverage for Part B purposes, you will be wrong and potentially subject to the late enrollment penalty. COBRA keeps your old insurance active, but Medicare does not treat it as current employer coverage.

People who are still working at 65 for a small employer (fewer than 20 employees) face a different coordination issue. In that case, Medicare becomes the primary payer even while you are still employed, meaning your employer plan pays second. This effectively makes Part B enrollment at 65 important even though you have employer coverage, because that employer plan will expect Medicare to pay first.

Part B for People Living With Disabilities Who Return to Work

One lesser-known provision protects people who qualified for Medicare through disability and then go back to work. If you lose your SSDI benefits because your earnings exceed the threshold for substantial gainful activity, your Medicare coverage can continue for a period under what is sometimes called the extended period of Medicare eligibility. During the trial work period and for a certain window after SSDI cash benefits stop, you keep your Part A and Part B coverage. This gives people with disabilities a bridge so that returning to work does not immediately strip them of health coverage, which would create a powerful disincentive to employment.

If you eventually lose Medicare eligibility entirely because you have sustained earnings above the disability threshold, and you later become unable to work again, you can re-apply for SSDI. If your condition is the same or related, you may be able to skip the five-month SSDI waiting period and the 24-month Medicare waiting period through expedited reinstatement provisions, though this process has its own rules and timelines. The point is that the system does have some flexibility built in, even if it is not always well publicized.

How Employer Size Affects Your Decision at 65

Whether your employer has 20 or more employees changes the calculus. For employers with 20 or more workers, the group health plan is required to be the primary payer for active employees aged 65 and over. Medicare pays second. In this situation, you can safely delay Part B enrollment without penalty as long as you remain actively employed and covered by the employer’s plan. Once you stop working or lose that coverage, your eight-month Special Enrollment Period begins.

For employers with fewer than 20 employees, Medicare is the primary payer. Your employer plan becomes secondary and may not cover much if you have not enrolled in Part B. In practice, this means working for a small company at age 65 without enrolling in Part B could leave you underinsured, because your employer plan expects Medicare to pick up the primary share and will only pay its portion after Medicare has paid first. If you have not signed up for Part B, there is no primary payer, and you could be stuck with large bills. This small-employer rule catches people by surprise more than almost any other aspect of Medicare eligibility.