Medicare Eligibility by Part: Who Qualifies for What

Why Medicare Eligibility Isn't One-Size-Fits-All

Most people hear "Medicare" and assume it works like a single program: turn 65, enroll, done. But Medicare is actually a collection of four distinct parts — A, B, C, and D — and each one has its own eligibility requirements, enrollment windows, and cost structure. Qualifying for one part doesn't automatically mean you qualify for, or are enrolled in, another.

This guide cuts through that complexity. Whether you're approaching 65, living with a qualifying disability, or helping a family member navigate their coverage options, you'll find the exact eligibility rules for each part laid out side by side — no bureaucratic fog, no buried fine print.

If you want a broader look at how the four parts work together day-to-day, see Your First Year on Medicare after you've worked through this reference. For now, let's focus on who gets in the door — and through which door.

Part A (Hospital Insurance): The Foundation

Part A is what most people consider the core of Medicare. It covers inpatient hospital stays, skilled nursing facility care, hospice, and some home health services. For the majority of beneficiaries, it's also premium-free — which makes its eligibility rules particularly important to understand.

Who Qualifies for Premium-Free Part A

You receive Part A without paying a monthly premium if any of the following apply:

• You are age 65 or older and you (or your spouse) paid Medicare taxes for at least 40 calendar quarters — that's 10 years of covered employment.
• You are under 65 and have received Social Security Disability Insurance (SSDI) benefits for 24 consecutive months.
• You have End-Stage Renal Disease (ESRD) — permanent kidney failure requiring dialysis or a transplant — regardless of age.
• You have Amyotrophic Lateral Sclerosis (ALS), also called Lou Gehrig's disease. ALS patients become eligible for Medicare the same month their SSDI benefits begin — the 24-month waiting period is waived entirely.

Who Pays a Part A Premium

If you don't meet the work-history threshold, you can still buy into Part A. The premium depends on how many quarters of coverage you've accumulated:

To purchase Part A, you must also be enrolled in Part B and meet the age or disability criteria above (minus the work-history piece).

One detail that trips people up: citizenship and residency matter. You must be a U.S. citizen or a lawfully admitted permanent resident who has lived in the U.S. continuously for at least five years before applying.

Part B (Medical Insurance): The Everyday Coverage

Part B handles what happens outside the hospital — doctor visits, outpatient procedures, preventive care, durable medical equipment, and certain home health services. Unlike Part A, Part B always carries a monthly premium, and enrollment isn't automatic for everyone.

Eligibility Requirements

Part B eligibility mirrors Part A on the major criteria:

• Age 65 or older and a U.S. citizen or qualifying permanent resident.
• Under 65 with SSDI — you're automatically enrolled in both Parts A and B after 24 months of disability benefits.
• ESRD or ALS — same rules as Part A apply.

The key difference: Part B is optional. You can decline it, though doing so without having other creditable coverage will trigger a late enrollment penalty — a 10% premium surcharge for each 12-month period you went without Part B when you were eligible.

The Late Enrollment Penalty

This penalty is permanent and follows you for life. It's one of the most costly mistakes Medicare-eligible individuals make. The only way to avoid it is to have creditable coverage through an employer or union group health plan that covers as much as Medicare does. When that coverage ends, you have a Special Enrollment Period to join Part B without penalty.

For more on how life changes create enrollment opportunities, the Special Enrollment hub explains qualifying events in detail.

Income-Based Premiums (IRMAA)

Higher-income beneficiaries pay more for Part B. The standard 2024 premium is $174.70 per month, but individuals earning above $103,000 (or couples above $206,000) pay an Income-Related Monthly Adjustment Amount (IRMAA) surcharge on top of that. CMS uses your tax return from two years prior to calculate your IRMAA bracket.

Part C (Medicare Advantage): A Different Path In

Part C — officially called Medicare Advantage — isn't a separate government program. It's a way to receive your Medicare benefits through a private insurer approved by CMS, rather than through the federal government directly. Plans often bundle Parts A, B, and D into a single product and may add extras like dental, vision, and hearing.

Who Is Eligible

Eligibility for Medicare Advantage is straightforward: you must already be enrolled in both Part A and Part B. That's it. If you meet the requirements for original Medicare, you can choose a Medicare Advantage plan instead — as long as one is offered in your geographic area.

Additional conditions that must be met:

• You must live in the plan's service area.
• You cannot have ESRD — with one significant exception: starting January 1, 2021, the 21st Century Cures Act allows people with ESRD to enroll in Medicare Advantage plans.
• You must continue paying your Part B premium (and your Part A premium, if applicable).

Enrollment Windows

You can join, switch, or drop a Medicare Advantage plan during specific enrollment periods:

• Initial Coverage Election Period (ICEP): When you first become eligible for Medicare.
• Annual Enrollment Period (AEP): October 15 – December 7 each year.
• Medicare Advantage Open Enrollment Period: January 1 – March 31, for people already in a Medicare Advantage plan.

Once you understand eligibility, the harder question becomes whether Medicare Advantage or Original Medicare better fits your situation. That tradeoff — including how Medigap fits in — is explored in Choosing the Right Combination of Medicare Parts.

Part D (Prescription Drug Coverage): Standalone or Bundled

Part D provides coverage for outpatient prescription drugs. You can get it one of two ways: as a standalone Prescription Drug Plan (PDP) added to Original Medicare, or bundled into a Medicare Advantage plan that includes drug coverage (MA-PD).

Who Is Eligible

Part D eligibility requirements are the same as Parts A and B:

• You must be enrolled in Part A or Part B (either one qualifies you — you don't need both).
• You must live in the plan's service area.
• You must be a U.S. citizen or qualifying permanent resident.

Unlike Part B, Part D is entirely voluntary. But declining it when you're first eligible — without having creditable drug coverage through another source like an employer plan or TRICARE — triggers a late enrollment penalty similar to Part B's: 1% of the national base beneficiary premium for every month you went without coverage, added permanently to your premium.

Low-Income Subsidy (Extra Help)

People with limited income and resources may qualify for the Low-Income Subsidy program, commonly called Extra Help. This program reduces or eliminates Part D premiums, deductibles, and copays. Eligibility is determined by the Social Security Administration based on income and asset limits — not by age or disability status alone.

It's worth noting that Part D plans vary significantly in which drugs they cover and what they charge — even within the same zip code. Using Medicare's Plan Finder tool at medicare.gov lets you compare plans based on your specific medications.

Side-by-Side Eligibility Comparison and Next Steps

Here's a consolidated view of how the four parts stack up on the major eligibility dimensions:

Common Scenarios Clarified

Scenario A — Turning 65 with no work history: You can purchase Part A (at full premium), enroll in Part B, and then access Part C or Part D. You won't get premium-free Part A, but you aren't excluded from the program.

Scenario B — Diagnosed with ESRD at age 42: You qualify for Parts A, B, C, and D immediately, regardless of work history or age — as long as you've applied through Social Security.

Scenario C — Retiring at 65 with employer coverage: You can delay Part B and Part D without penalty as long as your employer plan qualifies as creditable coverage. The moment that coverage ends, act quickly — your Special Enrollment Period is typically limited to 8 months for Part B.

Medicare vs. Medicaid: A Quick Distinction

Medicare eligibility is based primarily on age, disability, and work history. Medicaid, by contrast, is income-based and varies significantly by state. The two programs are frequently confused but serve different populations under different rules. For a direct comparison, see Medicaid vs. Medicare: Two Federal Programs, Very Different Rules. If you're trying to determine whether you might qualify for Medicaid instead of or alongside Medicare, Medicaid Eligibility Explained is your starting point.