When does Medicare eligibility begin?

Medicare eligibility generally begins at age 65 for US citizens and lawful permanent residents who have lived in the US for at least five years. People under 65 may also qualify if they have received Social Security Disability Insurance for a defined period or have certain qualifying conditions.

Most people do not pay a premium for Part A if they or their spouse worked and paid Medicare taxes for a sufficient period as defined by Social Security rules. People who do not meet the work requirement can buy into Part A coverage at a monthly premium set by CMS.

What is the difference between Original Medicare and Medicare Advantage?

Original Medicare (Parts A and B) is administered directly by the federal government and accepted by most providers nationwide. Medicare Advantage (Part C) is administered by private insurers approved by Medicare; it often includes drug coverage and may offer additional benefits, but typically restricts you to a specific network of providers.

What happens if I miss my initial enrollment window?

Missing the initial enrollment period for Part B or Part D can result in a late enrollment penalty — a permanent increase to your monthly premium. The penalty amount is calculated based on how long you were eligible but not enrolled. Exceptions apply if you had creditable coverage from another source.

Can I have both Medicare and Medicaid?

Yes. People who qualify for both programs are called dual eligible. Medicaid may cover costs that Medicare does not, including premiums, copays, and services not covered by Medicare. Coordination of benefits determines which program pays first.

What is the Medicare Initial Enrollment Period?

Your Initial Enrollment Period (IEP) is a seven-month window surrounding your 65th birthday: three months before, the month of, and three months after. Enrolling during the first three months of the IEP gives you the earliest possible coverage start date. Waiting until later in the window can delay when coverage begins. If you miss the IEP entirely and have no creditable coverage elsewhere, late enrollment penalties will apply.

What does Medicare NOT cover?

Original Medicare does not cover most routine dental care, routine vision or hearing exams and eyeglasses, long-term custodial care in a nursing home, cosmetic procedures, or most care received outside the United States. Some Medicare Advantage plans include limited dental, vision, or hearing benefits not available in Original Medicare. For services Original Medicare excludes, enrollees typically pay entirely out of pocket unless they have supplemental coverage.

What are the Medicare annual enrollment periods?

In addition to your Initial Enrollment Period, Medicare holds an Annual Enrollment Period from October 15 through December 7 each year. During this window you can switch between Original Medicare and Medicare Advantage, change Advantage plans, or change Part D plans. A Medicare Advantage Open Enrollment Period runs January 1 through March 31 and allows Advantage enrollees to switch plans or return to Original Medicare.

US Healthcare · System Basics

Medicare Basics

Medicare eligibility, Parts A through D, Medigap supplements, enrollment windows, late enrollment penalties, and how to verify a provider accepts Medicare.

Who this is for

People approaching age 65, individuals with qualifying disabilities, and family members helping someone navigate Medicare enrollment and coverage options.

Medicare is the federal health insurance program primarily serving people aged 65 and older, as well as certain younger individuals with qualifying disabilities or conditions. Administered by the Centers for Medicare and Medicaid Services (CMS), Medicare covers a broad range of healthcare services but involves premiums, deductibles, and copays — it is not free coverage. Understanding the different parts of Medicare, enrollment rules, and the options for supplemental coverage helps you make informed decisions at each stage of eligibility.

Medicare eligibility and who qualifies

The most common path to Medicare eligibility is turning 65. US citizens and lawful permanent residents who have lived in the US for at least five continuous years are eligible. You do not need to be retired — you can have Medicare while still working.

Younger people may also qualify for Medicare if they have received Social Security Disability Insurance (SSDI) payments for a period defined by federal rule. Additionally, individuals with certain conditions — end-stage renal disease (ESRD) requiring dialysis or a kidney transplant, or amyotrophic lateral sclerosis (ALS) — may qualify at any age.

If you already receive Social Security retirement benefits when you turn 65, you will generally be enrolled in Parts A and B automatically. If you are not yet receiving Social Security benefits, you must actively enroll during your Initial Enrollment Period — a seven-month window surrounding your 65th birthday.

The four parts of Medicare

Part A — Hospital insurance. Part A covers inpatient hospital care, skilled nursing facility (SNF) care following a qualifying hospital stay, hospice care, and some home health services. Most people do not pay a premium for Part A because they (or a spouse) paid Medicare taxes during their working years. Part A has deductibles and cost-sharing that apply per benefit period.

Part B — Medical insurance. Part B covers outpatient services, physician visits, preventive care, mental health services, and durable medical equipment (DME). Part B requires a monthly premium, which is set by CMS and adjusted annually. There is also an annual deductible, and after that, Part B typically covers a defined percentage of the approved amount for covered services.

Part C — Medicare Advantage. Medicare Advantage plans are offered by private insurers approved by Medicare and must cover everything that Original Medicare covers. Most Part C plans also include prescription drug coverage (Part D) and may offer additional benefits such as dental, vision, or hearing. Enrolling in Part C means your benefits are delivered through the private plan rather than directly through the federal government. These plans typically have provider networks, so the provider you want to see must participate in the plan.

Part D — Prescription drug coverage. Part D provides prescription drug coverage through standalone plans or through Medicare Advantage plans that include drug coverage. Part D plans have formularies — lists of covered drugs organized into tiers — with different cost-sharing at each tier. Plans vary significantly in which drugs they cover and at what cost. Enrolling in a Part D plan when first eligible and maintaining creditable drug coverage avoids late enrollment penalties.

Medigap supplemental insurance

Original Medicare (Parts A and B) leaves beneficiaries responsible for deductibles, coinsurance, and copays that can be substantial. Medigap (also called Medicare Supplement Insurance) policies, sold by private insurers, are designed to pay some or all of these remaining costs.

Medigap plans are standardized by federal law into lettered plan types (Plan G, Plan N, etc.), each with a defined set of benefits. You pay a monthly premium to the private insurer in addition to your Part B premium. Medigap plans do not include prescription drug coverage — you would need a separate Part D plan.

The best time to purchase a Medigap policy is during your Medigap open enrollment period, which begins when you turn 65 and enroll in Part B. During this window, insurers cannot deny you coverage or charge more based on your health status. Outside this window, medical underwriting may apply.

Late enrollment penalties

Missing enrollment deadlines for Parts B and D can result in permanent premium penalties. For Part B, the penalty is a percentage increase per full 12-month period you were eligible but not enrolled, with exceptions if you had qualifying employer coverage. For Part D, the penalty is calculated monthly and added to your premium for as long as you have Medicare drug coverage.

Exceptions exist when you delay enrollment because you have creditable coverage elsewhere — such as through an employer plan or a spouse’s plan. When you lose that coverage, a Special Enrollment Period allows you to enroll without penalty. Documenting your creditable coverage period is important.

Verifying provider participation

Not all providers accept Medicare. Providers fall into three categories: participating (accepts Medicare assignment for all patients), non-participating (can see Medicare patients but may charge more), and opted-out (does not accept Medicare at all). The Medicare Care Compare tool at medicare.gov lets you search for providers who accept Medicare in your area.

Comparing the four parts of Medicare

	Part A	Part B	Part C	Part D
What it covers	Inpatient hospital, skilled nursing facility, hospice, home health	Outpatient visits, physician services, preventive care, durable medical equipment	All Part A and B benefits via private insurer; often includes Part D	Prescription drugs via standalone plan or included in Advantage
Standard premium	$0 for most; up to several hundred dollars/mo without sufficient work history	Set annually by CMS; higher for high earners (IRMAA)	Varies by plan; may be $0 or a modest amount	Varies by plan; $0–$100+ per month
Annual deductible	Per benefit period (inpatient)	Annual deductible set by CMS	Varies by plan	Varies by plan
Provider access	Any Medicare-accepting provider nationwide	Any Medicare-accepting provider nationwide	Plan’s contracted network only	Plan’s contracted pharmacies
Prescription drugs	Not covered	Not covered	Included in most plans	Covered; formulary varies by plan

What this looks like in practice

Imagine Maria, who emigrated from the Philippines 20 years ago and became a US citizen. She turns 65 in September and is not yet collecting Social Security. She must actively enroll during her Initial Enrollment Period — July through January. She signs up for Part A (free, because she worked for 12 years and paid Medicare taxes) and Part B in August, so her coverage starts October 1.

Her doctor recommends a hip replacement. Under Original Medicare, Part A covers her inpatient hospital stay; she pays the Part A deductible for the benefit period. Part B covers her surgeon’s fees at 80% of the Medicare-approved amount after the Part B deductible — leaving her responsible for the 20% coinsurance with no annual cap on that liability.

Because the 20% coinsurance has no ceiling under Original Medicare, Maria purchases a Medigap Plan G policy. The Medigap policy covers her 20% Part B coinsurance after she pays the Plan G annual deductible. She also enrolls in a standalone Part D plan for her blood pressure and cholesterol medications.

Her total monthly cost is: Part B premium plus Medigap Plan G premium plus Part D premium. The costs are higher than having no insurance, but she has predictable expenses and no risk of a catastrophically large coinsurance bill. Had she waited until after her Initial Enrollment Period without creditable coverage, she would have faced a permanent Part B late enrollment penalty added to her premium every month for the rest of her life.

Step by step: how to enroll in Medicare

Confirm your eligibility start date — the month you turn 65 for most people, or the date you qualify through disability or a qualifying condition.
Determine whether you will be auto-enrolled. If you are already receiving Social Security or Railroad Retirement Board benefits, enrollment in Parts A and B is typically automatic. If not, you must actively apply.
Visit ssa.gov, call Social Security at 1-800-772-1213, or go to a local Social Security office to enroll in Part A and Part B during your Initial Enrollment Period.
Decide between Original Medicare and Medicare Advantage. If considering Advantage, research plans at medicare.gov/plan-compare, paying attention to network coverage, included drug formulary, and extra benefits.
If staying in Original Medicare, shop for a Medigap supplemental policy during your Medigap open enrollment period — the six months starting the month you are both 65 and enrolled in Part B. Insurers cannot deny coverage or charge more based on health status during this window.
Enroll in a Part D prescription drug plan through medicare.gov/plan-compare if your coverage does not already include drug coverage. Missing Part D enrollment without creditable drug coverage results in a permanent monthly penalty.
Verify that your current providers accept Medicare before scheduling your first appointments by searching medicare.gov/care-compare.
Review your Medicare Summary Notice or Explanation of Benefits after each service to confirm billing accuracy and track progress toward deductibles.

Documents and terms you’ll see

When navigating Medicare enrollment and benefits, you will encounter the following terms in official correspondence, enrollment notices, and Explanation of Benefits forms:

Premium — the monthly amount you pay to maintain Medicare coverage, separate from what you pay when you receive care

Deductible — the amount you pay for covered services before Medicare begins sharing costs; Part A and Part B each have their own separate deductibles

Coinsurance — your share of costs after the deductible; under Part B this is typically 20% of the Medicare-approved amount with no annual cap under Original Medicare

Medicare Part A — the hospital insurance component covering inpatient stays, skilled nursing care, hospice, and home health services

Medicare Part B — the medical insurance component covering outpatient services, physician visits, preventive care, and durable medical equipment

Medicare Supplement — also called Medigap; private insurance sold by approved insurers that fills the cost gaps left by Original Medicare

Dual eligible — a person who qualifies for both Medicare and Medicaid simultaneously; Medicaid may cover premiums and cost-sharing that Medicare does not pay

Key terms

Term	Plain meaning	Glossary
Medicare Part A	Covers inpatient hospital stays, skilled nursing facility care, hospice, and some home health	→
Medicare Part B	Covers outpatient services, doctor visits, preventive care, and durable medical equipment	→
Medicare Part C	Medicare Advantage — private plans that deliver Medicare benefits and often include Part D	→
Medicare Part D	Prescription drug coverage, available as a standalone plan or through Medicare Advantage	→
Medicare Supplement	Medigap — private insurance that helps pay costs Original Medicare does not fully cover	→

Common questions

When does Medicare eligibility begin?: Medicare eligibility generally begins at age 65 for US citizens and lawful permanent residents who have lived in the US for at least five years. People under 65 may also qualify if they have received Social Security Disability Insurance for a defined period or have certain qualifying conditions.
Is Part A free?: Most people do not pay a premium for Part A if they or their spouse worked and paid Medicare taxes for a sufficient period as defined by Social Security rules. People who do not meet the work requirement can buy into Part A coverage at a monthly premium set by CMS.
What is the difference between Original Medicare and Medicare Advantage?: Original Medicare (Parts A and B) is administered directly by the federal government and accepted by most providers nationwide. Medicare Advantage (Part C) is administered by private insurers approved by Medicare; it often includes drug coverage and may offer additional benefits, but typically restricts you to a specific network of providers.
What happens if I miss my initial enrollment window?: Missing the initial enrollment period for Part B or Part D can result in a late enrollment penalty — a permanent increase to your monthly premium. The penalty amount is calculated based on how long you were eligible but not enrolled. Exceptions apply if you had creditable coverage from another source.
Can I have both Medicare and Medicaid?: Yes. People who qualify for both programs are called dual eligible. Medicaid may cover costs that Medicare does not, including premiums, copays, and services not covered by Medicare. Coordination of benefits determines which program pays first.
What is the Medicare Initial Enrollment Period?: Your Initial Enrollment Period (IEP) is a seven-month window surrounding your 65th birthday: three months before, the month of, and three months after. Enrolling during the first three months of the IEP gives you the earliest possible coverage start date. Waiting until later in the window can delay when coverage begins. If you miss the IEP entirely and have no creditable coverage elsewhere, late enrollment penalties will apply.
What does Medicare NOT cover?: Original Medicare does not cover most routine dental care, routine vision or hearing exams and eyeglasses, long-term custodial care in a nursing home, cosmetic procedures, or most care received outside the United States. Some Medicare Advantage plans include limited dental, vision, or hearing benefits not available in Original Medicare. For services Original Medicare excludes, enrollees typically pay entirely out of pocket unless they have supplemental coverage.
What are the Medicare annual enrollment periods?: In addition to your Initial Enrollment Period, Medicare holds an Annual Enrollment Period from October 15 through December 7 each year. During this window you can switch between Original Medicare and Medicare Advantage, change Advantage plans, or change Part D plans. A Medicare Advantage Open Enrollment Period runs January 1 through March 31 and allows Advantage enrollees to switch plans or return to Original Medicare.

Sources

Last reviewed: September 2026

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