Williams | Robinson | Wiggins

Enrollment deadlines and penalties, what Original Medicare doesn't cover, and choosing between a Medicare Supplement and Medicare Advantage.

Turning 65 triggers one of the more consequential enrollment decisions most people will face, and Medicare's own vocabulary is often the first obstacle. A typical health insurance policy bundles hospital, doctor, and prescription coverage under one plan. Medicare instead splits coverage into separate "Parts," each with its own rules, its own timeline, and — in some cases — a permanent penalty for enrolling late. This article is not intended to tell a reader which option to choose, but to provide enough background to intelligently compare the options and ask the right questions. The forms mentioned in this article are linked below.

Reviewed through July 2026. Dollar figures are for calendar year 2026 unless noted; premiums, plan availability, networks, and enrollment rules change annually.

The vocabulary is the first hurdle. Medicare's coverage is broken into "Parts," and some Parts carry additional common names:

• Medicare Part A — most hospital and inpatient coverage
• Medicare Part B — physician services, outpatient care, diagnostic testing, durable medical equipment, preventive services, and certain outpatient drugs
• Original Medicare — another name for Parts A and B together
• Medicare Part D — prescription drug coverage, sold by private insurers
• Medicare Supplement / Medigap — private supplemental coverage that helps pay certain deductibles, coinsurance, and co-pays for services covered by Original Medicare
• Medicare Advantage / Medicare Part C — a Medicare-approved private plan that provides an alternative way to receive Part A and B benefits, and usually includes Part D
• HMO / PPO — two common network structures used by Medicare Advantage plans
• In-Network — a hospital or physician group that has contracted with a particular insurer

The letters used for Medigap Plans A through N (Plan G, Plan N, and so on) are a separate naming system from Medicare's own Parts A through D. In most states, each letter identifies a benefit package standardized by law — the benefits within a given lettered plan are identical from carrier to carrier, though premiums are not. (Massachusetts, Minnesota, and Wisconsin standardize their plans differently.)

Enrollment is tied to turning 65, not to full Social Security retirement age. Social Security full retirement age is gradually rising toward 67, but most people first become eligible for Medicare at 65 and should enroll during their Initial Enrollment Period unless they have a valid reason to delay, usually qualifying coverage based on the current employment of the enrollee or a spouse — someone born in 1962, for example, would ordinarily enroll in Medicare at 65 in 2027, two years before reaching Social Security full retirement age in 2029. The Initial Enrollment Period runs 7 months: it opens 3 months before the birthday month, includes the birthday month itself, and stays open for 3 months after. Enrollment in Original Medicare is generally automatic for anyone already drawing Social Security benefits before 65. Someone still working at 65 with active employer coverage generally does not have to enroll — but only if the employer has 20 or more employees; under Medicare's "Small Employer" exception, when the employer has fewer than 20 employees Medicare pays first, so the person should generally enroll in both premium-free Part A and Part B — Part A alone does not cover physician and outpatient services, and the employer plan may calculate its benefits as though Medicare had paid even if the person never enrolled. Because the Medicare Secondary Payer rules for the working-aged do not bind employers of this size, the plan may also, depending on its terms, reduce or decline to continue coverage. (Certain multi-employer plans that include at least one employer of 20 or more are treated differently.) When in doubt about whether an employer plan qualifies, confirm the coordination rules with the employer's benefits administrator and with Medicare or Social Security before deciding to delay or to enroll.

People entitled to Social Security disability benefits are also enrolled automatically, though not immediately — Medicare generally begins after 24 months of entitlement to disability benefits, which may itself follow a separate 5-month waiting period for the disability benefits. Two conditions receive special treatment: End-Stage Renal Disease has its own Medicare eligibility rules, and a person entitled to disability benefits because of ALS receives Medicare without the usual waiting periods. Other medical conditions can still lead to Medicare through a disability award — but only after the waiting period above.

Missing the window can carry a lifetime penalty. There is no penalty for enrolling in Part A late if it is premium-free. But someone who turns 65 without active employer coverage and does not enroll in Part B on time faces a permanent penalty of 10 percent of the premium for each full 12-month period enrollment was delayed — added to the premium for as long as that person has Part B, unless a Special Enrollment Period or other exception applies. The safety valve: someone who properly delays Part B because of coverage based on current employment gets an 8-month Special Enrollment Period to enroll penalty-free, starting the month the employment or the coverage ends, whichever comes first — and COBRA or retiree coverage does not extend that deadline. Part D carries a similar penalty if a person goes 63 consecutive days or more without "creditable" prescription coverage after becoming eligible: 1 percent of the national base premium (set at $38.99 for 2026) for each full uncovered month. A 12-month uncovered period, for example, produces a 12 percent penalty calculated against each year's national base premium — a dollar amount that can change annually — for as long as the person has Part D. One more planning point worth flagging early: Health Savings Account (HSA) contributions must stop before Medicare coverage begins, and a person applying for premium-free Part A after 65 should generally stop contributing at least 6 months before applying, because Part A can be made retroactive for up to 6 months.

Original Medicare, by itself, leaves significant gaps. Unlike most health insurance, Original Medicare has no annual out-of-pocket maximum. For 2026, Part A inpatient care carries a $1,736 deductible per benefit period (after which days 1–60 carry no daily hospital coinsurance), then a $434-per-day coinsurance for days 61–90, and an $868-per-day coinsurance for 60 "lifetime reserve" days that, once used, cannot be used again. Skilled nursing under Part A, when Medicare's eligibility and qualifying-stay requirements are satisfied, is covered in full for the first 20 days, then carries a $217-per-day coinsurance through day 100, after which Medicare pays nothing. Part B carries a $283 annual deductible, after which Medicare pays 80 percent of the Medicare-approved amount for most covered services — leaving the enrollee responsible for the other 20 percent indefinitely, again with no annual cap. Because of this gap, most people choose between two paths — keeping Original Medicare and adding coverage (Medigap plus Part D), or receiving Medicare benefits through a Medicare Advantage plan instead — and only one of the two can be chosen, not both. (Some people instead have employer, retiree, union, TRICARE, VA, or Medicaid coverage that fills the same role.)

Path #1: Medicare Supplement (Medigap), paired with a separate Part D plan. A Medigap policy is purchased from a private insurer to pay some or all of the costs Original Medicare leaves behind. Medigap plans are standardized by letter — for example, Plan G (among the most popular, and covering nearly all out-of-pocket cost other than the Part B deductible) or Plan N (a lower premium in exchange for co-pays on doctor and ER visits). Medigap does not include drug coverage, so an enrollee who wants drug coverage adds a Part D plan purchased separately from a private carrier — technically optional, but going without creditable drug coverage triggers the penalty above. Since 2025, out-of-pocket spending on covered Part D drugs has been capped by law — $2,100 for 2026 — after which the enrollee pays nothing further for covered drugs that calendar year, whichever path is chosen. Because Original Medicare lets enrollees see any provider nationwide who accepts Medicare — and a standard Medigap policy follows that coverage anywhere (Medicare SELECT policies, which use networks, are the exception) — this path tends to produce the most predictable, if higher, monthly cost. Medigap premiums vary widely by plan, location, age, and rating method (the illustration below assumes $180 per month for a Plan G), while stand-alone Part D plans average about $34.50 per month for 2026. Part D premiums in particular are worth confirming directly: CMS announced in July 2026 that its voluntary Part D Premium Stabilization Demonstration for participating stand-alone plans will end after 2026, and the 2027 national base premium will rise from $38.99 to $41.33, so 2027 premiums are expected to shift from the figures above — check current pricing at Medicare.gov or with a carrier before enrolling.

Path #2: Medicare Advantage (Medicare Part C). Medicare Advantage plans are private, Medicare-approved plans that provide an alternative way to receive Part A and B benefits (and, in most policies, Part D too) — the enrollee remains enrolled in Medicare and keeps paying the Part B premium — often bundling in benefits Original Medicare does not cover at all, such as dental, vision, and hearing. Because private carriers administer most covered benefits under Medicare's rules, premiums are frequently very competitive — some plans charge no premium beyond the standard Part B premium. In exchange, enrollees are generally required to use "in-network" providers, and must choose between two network structures:

• HMO (Health Maintenance Organization) — managed care through a defined network; a Primary Care Physician referral is usually required to see most specialists, and out-of-network care is generally not covered except in an emergency (some HMO-POS variants cover limited out-of-network care). HMO plans often carry lower cost-sharing in exchange for tighter restrictions, with a more limited ability to choose physicians than in a PPO.
• PPO (Preferred Provider Organization) — much broader access to providers, usually no referral required to see a specialist, and out-of-network care is still paid, though at a reduced benefit level. PPO plans tend to suit people in rural areas where specialists are spread across different health systems.

Two more Medicare Advantage features belong in any comparison. These plans commonly require prior authorization (advance plan approval) for certain services, which Original Medicare generally does not. On the other side of the ledger, every Medicare Advantage plan must cap the enrollee's annual in-network out-of-pocket costs for covered medical care — at most $9,250 for 2026, with many plans setting lower limits — a protection Original Medicare alone lacks. Each plan's Evidence of Coverage controls the details.

Weighing the two paths. Neither path is universally "better" — the right choice depends on health needs, budget, and location. As a rough illustration for 2026: an enrollee combining Original Medicare with a Part D plan and a Medigap Plan G can expect a consistent, predictable premium of around $417.40 per month in this illustration ($202.90 for Part B, plus a $34.50 average Part D premium and a $180 Plan G premium), or roughly $5,292 per year (which also includes the $283 Part B annual deductible), before any drug co-pays. A Medicare Advantage plan, by contrast, carries a highly variable annual cost — as low as the $202.90 Part B premium alone (about $2,435 per year) for a $0-premium plan with minimal co-pays, up to or exceeding what a Medigap/Part D combination would cost if the enrollee needs significant care. In short: Medigap trades a higher, steady monthly premium for freedom of provider choice and predictability; Medicare Advantage trades a typically lower up-front cost for network restrictions and less predictable co-pays. For the 2026 plan year, multiple carriers offer $0-premium Medicare Advantage plans in Phelps County, Missouri, so availability is rarely the limiting factor — the comparison usually comes down to whether predictable bills or a lower up-front cost matters more for a given enrollee. Plan availability, premiums, and networks are set county by county and change every plan year; check the current offerings for any county at Medicare Plan Compare (medicare.gov/plan-compare).

A few other paths are worth knowing about. A Chronic Condition Special Needs Plan (C-SNP) is a specialized Medicare Advantage plan available only to people diagnosed with certain qualifying chronic conditions. A Dual Special Needs Plan (D-SNP) serves people eligible for both Medicare and Medicaid under the plan's eligibility rules, and coordinates the two programs' benefits — some plans more fully integrated than others. Both are simply specialized categories of Medicare Advantage, generally organized as HMO, HMO-POS, or PPO plans depending on what is available locally.

Changing course later. A Medigap application can be submitted at any time, but the guaranteed window is front-loaded: each person gets a one-time, 6-month Medigap Open Enrollment Period, beginning when they are 65 or older and first enrolled in Part B, during which no medical underwriting applies. Outside that window (or a specific guaranteed-issue right), an insurer may underwrite, charge more, or decline the application — so an existing policy should never be cancelled until the replacement is in force. A Part D plan generally may be changed during the Annual Enrollment Period (October 15 – December 7), with additional opportunities in specified circumstances. Medicare Advantage plans can be changed during that same Annual Enrollment Period, or during the separate Medicare Advantage Open Enrollment Period (January 1 – March 31). Moving to a new area, losing other coverage, or certain other qualifying changes can open a Special Enrollment Period outside the usual windows — but each Special Enrollment Period is event-specific: the triggering event determines which changes are allowed, and a Medicare Advantage or Part D Special Enrollment Period does not automatically create a Medigap guaranteed-issue right.

Summary. Enrolling in Medicare Part A and Part B is the starting point, but Original Medicare alone leaves no out-of-pocket cap and real gaps in coverage. Filling those gaps means choosing between a Medicare Supplement (Medigap) paired with a separate Part D plan, or a bundled Medicare Advantage (Part C) plan — not both. Missing the initial enrollment window for Part B or Part D can mean a permanent penalty added to the premium for life, so this decision is worth researching well before the enrollment window opens, not during it.

For free, unbiased, one-on-one help comparing plans, Missouri's State Health Insurance Assistance Program (SHIP) — missouriship.org, or (800) 390-3330 — provides certified counselors at no cost who do not sell insurance and have nothing to gain from any particular recommendation.

Did you know? The above information is presented by Williams | Robinson | Wiggins as a public service and to generally outline the law in a particular area. It is not provided and is not intended as legal advice tailored to you or to your unique situation. Every legal matter depends upon specific facts which an attorney hired by you must consider in forming legal opinions and advice.

Need more information? If you have questions about how a Medicare decision may intersect with your broader estate or elder-care planning, you may contact us at(573) 341-2266 to obtain more detailed assistance. Please feel free to download a more detailed treatment of this subject, our Medicare Enrollment Guide (.PDF), from our Forms section.

If you already have Part A and are ready to add Part B — whether during your Initial Enrollment Period, the General Enrollment Period, or a Special Enrollment Period — Social Security requires this signed request. (Someone enrolling in Medicare for the first time instead applies through Social Security directly, online or at a local office.)

Medicare Part B Enrollment Request (CMS-40B)

If you're relying on current or prior employer group coverage to enroll during a Special Enrollment Period without a late-enrollment penalty, your employer completes Section B of this companion form confirming that coverage. (If the employer cannot, Social Security permits you to complete it as best you can and submit other proof of the coverage.)

Request for Employment Information (CMS-L564)

Both forms are also available for download anytime — click the "☰" icon on the navigation bar and select "Forms."