Medicare Enrollment Timeline: When to Sign Up

Medicare gives you a 7-month window to enroll at 65. Miss it and you pay a permanent premium surcharge for the rest of your life. Here's exactly when to enroll, what the penalties cost, and when waiting is actually fine.

The 7-Month Initial Enrollment Period (IEP)

When you first become eligible for Medicare at 65, you have a 7-month Initial Enrollment Period to sign up for Part A and Part B:¹

• Months 1–3: The 3 months before your birthday month
• Month 4: Your birthday month
• Months 5–7: The 3 months after your birthday month

After this window closes, your next opportunity is the General Enrollment Period (January 1 – March 31), but you'll owe a late enrollment penalty from that point forward.

When Coverage Actually Starts: The Timing Trap

Enrolling later in your IEP doesn't just mean starting coverage later — it creates a gap. The month you enroll determines when Part B begins:¹

Practical takeaway: Enroll in the first 3 months of your IEP (before your birthday month) so coverage starts the first of your birthday month with no gap. You get the same benefit at no extra cost — there's no reason to wait.

Part B Late Enrollment Penalty: 10% Per Year, Permanent

If you miss your IEP without a valid exception, Medicare adds a permanent surcharge to your Part B premium for each full 12-month period you went without coverage:²

• Penalty rate: 10% × number of 12-month periods without Part B
• 2026 Part B base premium: $202.90/month³
• Duration: Permanent — you pay it for as long as you have Part B

Note: The 10% penalty applies to each 12-month period, not each month. Three years late = 30%, not 36%.

Part D Late Enrollment Penalty: 1% Per Month, Also Permanent

Medicare drug coverage (Part D) has a separate penalty for gaps in creditable coverage:²

• Trigger: Any gap of 63 or more consecutive days without Part D or qualifying creditable drug coverage
• Penalty formula: 1% × base beneficiary premium ($38.99 in 2026⁴) × number of uncovered months
• Duration: Permanent, and it recalculates upward each year as the base premium rises

An employer drug plan, TRICARE, or VA coverage can qualify as creditable — get it confirmed in writing before your IEP ends. Keep that documentation; SSA can ask for it when you eventually enroll.

The General Enrollment Period: Fallback if You Miss Your IEP

If you miss your IEP and don't qualify for a Special Enrollment Period, you can sign up during the General Enrollment Period (GEP) that runs January 1 – March 31 every year. Coverage starts July 1.¹

The catch: you'll owe the Part B late enrollment penalty for every full year you waited, starting immediately and lasting forever. The 3-to-4 month gap between enrollment and coverage start (January–March → July 1) also means months without Part B outpatient coverage unless you have other insurance.

Still Working at 65? Your IEP Clock Pauses

If you're covered by an active group health plan through an employer with 20 or more employees, you can delay Part B enrollment without penalty. When your employer coverage ends — at retirement, job loss, or plan termination — you get an 8-month Special Enrollment Period to enroll in Part B with no late penalty and no gap.⁵

Two critical traps for workers over 65:

• Small employer (<20 employees): Medicare becomes primary payer regardless of employer coverage. You must enroll in Part B at 65 or face penalties.
• COBRA doesn't count: COBRA is not active employer coverage. The SEP starts when you lose your group plan, not when COBRA ends. Don't wait out COBRA — you may miss the 8-month window.

Full analysis of the still-working decision: Medicare at 65 with Employer Coverage →

Annual Enrollment Period: Changing Plans Each Fall

Once enrolled, the Annual Enrollment Period (AEP) runs October 15 – December 7 each year. During this window you can:¹

• Switch between Original Medicare and Medicare Advantage
• Change Medicare Advantage plans
• Join, switch, or drop a Part D plan

Changes take effect January 1. Outside the AEP, you can only change plans with a qualifying Special Enrollment Period (moved out of plan area, lost coverage, gained Low Income Subsidy). The MA → Medigap switch after the AEP is subject to medical underwriting in most states, which effectively locks in your Medigap decision for beneficiaries with health conditions.

Full comparison: Medicare Advantage vs Medigap →

The IRMAA Two-Year Planning Window

Your Part B and Part D premiums are based on your MAGI from two years prior. If you enroll in Medicare in 2026, your 2026 premiums are set by your 2024 tax return. This creates a look-back that matters for pre-retirement planning:

• Large Roth conversions or capital gain realizations at age 63–64 will directly set your first 1–2 years of IRMAA surcharges
• If your income drops sharply when you retire, file Form SSA-44 to request IRMAA be calculated on current-year income — potentially saving $1,148–$6,936/year per person
• Planning Roth conversions and capital-gain harvesting before age 65 requires modeling the two-year look-back alongside your retirement income projections

Deep dive: Roth Conversions and IRMAA → | How to Appeal IRMAA Surcharges → | IRMAA Bracket Calculator →

Sources

1. Medicare.gov — When can I sign up for Medicare? (IEP, GEP, AEP windows)
2. Medicare.gov — Avoid Late Enrollment Penalties (Part B 10% formula; Part D 1% formula)
3. CMS — 2026 Medicare Parts A & B Premiums and Deductibles (Part B standard monthly premium $202.90; Part B annual deductible $283)
4. CMS — 2026 Medicare Part D Bid Information (national base beneficiary premium $38.99)
5. SSA.gov — When to Sign Up for Medicare (Special Enrollment Period for employer coverage)

Premium values verified against CMS final rules published November 2025. Penalty formulas per 42 U.S.C. § 1839(b) (Part B) and 42 U.S.C. § 1860D-13(b) (Part D). Values current as of April 2026.