What Is Medicare and When Should You Enroll?

Medicare is the federal health insurance program for Americans 65 and older — but the enrollment rules are strict, the parts are confusing, and missing your window can mean permanent penalties. Here's exactly what Medicare covers, when to sign up, and how to avoid the most common (and costly) mistakes.

What Is Medicare?

Medicare is federal health insurance primarily for people 65 and older, though it also covers some younger people with certain disabilities or conditions like End-Stage Renal Disease. Unlike Medicaid, Medicare isn't income-based — eligibility is based on age and work history, not financial need.

Medicare is split into several parts, each covering something different:

Part	What It Covers
Part A	Hospital insurance — inpatient stays, skilled nursing, hospice
Part B	Medical insurance — doctor visits, outpatient care, preventive services
Part C	Medicare Advantage — private plans that bundle Parts A, B, and usually D
Part D	Prescription drug coverage

How Much Does Medicare Cost?

• Part A: Usually $0 premium if you (or a spouse) paid Medicare payroll taxes for at least 10 years (40 quarters). Otherwise, up to $518/month in 2026.
• Part B: $185/month standard premium in 2026, though higher earners pay more (see IRMAA below).
• Part D: Varies by plan, typically $30–$60/month.
• Medicare Advantage (Part C): Often $0 premium beyond Part B, but with its own deductibles, copays, and provider networks.

When Should You Enroll in Medicare?

This is where most of the costly mistakes happen. Your Initial Enrollment Period is a 7-month window centered on your 65th birthday:

• 3 months before the month you turn 65
• The month you turn 65
• 3 months after the month you turn 65

If you're already receiving Social Security benefits when you turn 65, you're automatically enrolled in Parts A and B — no action needed. If you haven't yet claimed Social Security, you need to actively sign up through the Social Security Administration.

What Happens If You Miss Your Enrollment Window?

Miss it, and you face two problems: a gap in coverage, and permanent late penalties.

Part	Late Enrollment Penalty
Part B	10% added to premium for each full 12-month period you delayed — permanently, for as long as you have Part B
Part D	1% of the national base premium for each month without creditable drug coverage — permanently

These penalties don't expire — they're added to your premium for as long as you have that coverage, which for most people means the rest of their life.

The Big Exception: Still Working With Employer Coverage

If you're still working at 65 and covered by a qualifying employer health plan (generally, an employer with 20 or more employees), you can delay Parts B and D without penalty. When you eventually leave that job or lose that coverage, you get a Special Enrollment Period — 8 months to sign up for Part B, and 2 months for Part D — without facing late penalties.

This exception does not apply to small employers with fewer than 20 employees, COBRA coverage, or retiree health plans — those don't count as "creditable coverage" for delaying Medicare, and delaying based on them can trigger penalties.

What Is IRMAA?

IRMAA (Income-Related Monthly Adjustment Amount) is a surcharge added to Part B and Part D premiums for higher earners, based on your Modified Adjusted Gross Income from two years prior.

2024 Income (Single)	2026 Part B Monthly Premium
$106,000 or less	$185.00 (standard)
$106,000 – $133,000	$259.00
$133,000 – $167,000	$370.00
Over $500,000	$629.90

Because IRMAA looks back two years, a large income event — like a Roth conversion or a big capital gain — at age 63 can increase your Medicare premiums at 65. This is one reason retirement income planning (including Roth conversions) is often done with IRMAA thresholds in mind.

Medicare Advantage vs. Original Medicare

	Original Medicare (A + B)	Medicare Advantage (Part C)
Provider network	Any doctor accepting Medicare, nationwide	Limited to plan network, usually regional
Extra benefits	None (needs separate Medigap + Part D)	Often includes dental, vision, hearing
Out-of-pocket cap	None, unless you add Medigap	Yes, annual maximum required by law
Referrals needed	No	Often yes, for specialists

Neither option is universally better — it depends on how much you travel, whether you have preferred doctors outside a network, and how much predictability you want in your out-of-pocket costs.

What About Medigap?

Medigap (Medicare Supplement Insurance) is private insurance that fills the "gaps" in Original Medicare — deductibles, copays, and coinsurance. It only works alongside Original Medicare, not Medicare Advantage. The best time to buy a Medigap policy is during your 6-month Medigap Open Enrollment Period, which starts the month you're both 65 and enrolled in Part B — during this window, insurers can't deny you coverage or charge more based on health conditions. Miss it, and medical underwriting may apply.

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Common Medicare Mistakes to Avoid

• Missing your Initial Enrollment Period — leads to permanent late penalties on Part B and/or Part D.
• Assuming small-employer or COBRA coverage lets you delay penalty-free — it generally doesn't; only larger employer group plans qualify.
• Not planning around IRMAA — a large income event two years before enrolling can unexpectedly raise your premiums.
• Delaying a Medigap purchase — missing the 6-month open enrollment window can mean medical underwriting and higher costs, or denial.
• Not comparing Medicare Advantage vs. Original Medicare annually — plans and costs change every year during Open Enrollment (October 15 – December 7).

Bottom Line

Medicare enrollment isn't automatic for everyone, and the penalties for missing your window are permanent. If you're approaching 65, mark your Initial Enrollment Period on the calendar well ahead of time, understand whether your current employer coverage lets you delay penalty-free, and factor IRMAA into any large income decisions in the years just before you enroll.