When to Sign Up for Medicare: A Complete Enrollment Timeline

If you’re still working at 65 and have employer-sponsored health insurance, you can delay Medicare Part B and Part D without paying a late penalty — but only if the employer plan is “creditable” and the company has 20 or more employees. The key rule: you must sign up for Part B and Part D during a Special Enrollment Period (SEP) that starts the month after you stop working or lose coverage, whichever comes first. For Part B, the SEP lasts 8 months; for Part D, you get 2 months after losing creditable drug coverage. Miss that window, and the late enrollment penalty applies permanently.

Here’s what that means for your next move: if your employer has at least 20 employees and your drug plan is creditable, you can safely delay Part B and Part D until you retire. If not, you must enroll during your Initial Enrollment Period at 65 or face permanent late penalties. Your first action step is to confirm your employer’s group size and get written proof of creditable coverage — do this now, even if retirement is years away.

This page walks you through exactly when to sign up, the most common mistake people make, and the concrete steps you need to avoid penalties.

When You Can Safely Delay Medicare

Medicare recognizes that if you have job-based insurance from a current employer (or your spouse’s employer) with at least 20 employees, you don’t need to sign up for Part B or Part D during your Initial Enrollment Period (IEP) at age 65. You can wait until you retire.

What Qualifies as Creditable Coverage for Part D

Your employer must certify that your drug plan is at least as good as Medicare’s standard Part D coverage. If they don’t, you may need to sign up for Part D at 65 or face a penalty later. The employer must provide a Creditable Coverage Notice annually — keep this letter in your files.

Part A Is Different

Most people get Part A premium-free (if they paid Medicare taxes for 10+ years). You can enroll in Part A at 65 even while working, with no penalty. Many people do this because Part A covers inpatient hospital care and doesn’t conflict with employer insurance. However, if you have a Health Savings Account (HSA), enrolling in any part of Medicare stops HSA contributions after the month you enroll. If you want to keep contributing to your HSA, delay Part A as well.

The SEP Timeline

Once you stop working (or your employer coverage ends), you have an 8-month window to sign up for Part B without penalty. For Part D, the SEP is 2 months from losing creditable drug coverage. You can use both SEPs at the same time.

Example: Maria turns 65 in January 2025, works full-time for a company with 200 employees, and has good employer coverage. She delays Part B and Part D. She retires in June 2025. Her SEP for Part B begins July 2025 and ends February 2026. She must sign up for Part B within those 8 months to avoid a penalty. For Part D, if her employer drug coverage ends June 2025, she must sign up by August 2025.

The Big Mistake: Assuming Any Employer Plan Qualifies

The most common failure mode is assuming all job-based insurance allows you to delay Medicare. It does not. The rule only applies if your employer (or your spouse’s employer) has 20 or more employees. If the company has fewer than 20 employees, Medicare becomes your primary payer, and you must enroll in Part A and Part B during your IEP – or face permanent late enrollment penalties.

How to Detect This Early

Ask your HR department: “Does our company have 20 or more full-time equivalent employees? And can you confirm my drug coverage is creditable for Medicare Part D?” Get it in writing — a letter or email with your company’s headcount and a Creditable Coverage Notice. If the answer is “fewer than 20,” sign up for Part B and Part D at 65.

Real-World Penalty Example

John turns 65 in April 2024. He works for a small business with 12 employees. He decides not to sign up for Part B because he has employer coverage. He retires in April 2026, two years later. He tries to use the SEP, but his employer had fewer than 20 employees, so the SEP doesn’t apply. He is now two full 12-month periods (24 months) late. The Part B late enrollment penalty is 10% of the standard premium for each full 12-month period. In 2025, the standard Part B premium is $185.00/month. John’s penalty would be 20% × $185.00 = $37.00 extra per month, added to his premium for life.

Other Pitfalls

COBRA coverage does not count as “current employer coverage” for the SEP. If you retire and take COBRA, you cannot delay Part B based on that. You must sign up for Part B during the 8-month SEP that starts when your employment ended, not when COBRA ends. If you miss that window, the late penalty applies even if you still have COBRA.

Retiree insurance also does not qualify for the SEP. Only active employer group health plans allow the delay. If you switch to a retiree plan after leaving work, the 8-month SEP clock already started ticking from your last day of active employment.

Your Enrollment Timeline: Step by Step

Step 1: Confirm employer size and creditable coverage

Contact HR or benefits administrator. Ask:

• Does the company have 20+ employees?
• Is the health plan considered creditable for Part D?

Get written confirmation (a letter or an email). Keep it with your records. If you cannot get a letter, save your pay stubs showing employer name and your most recent Summary of Benefits and Coverage.

Step 2: Decide on Part A

You can enroll in Part A at any time after turning 65, even while working. There is no penalty for late Part A if you are premium-free. Many people take Part A at 65 to get hospital coverage, but be aware: enrolling in Medicare stops the ability to contribute to an HSA after the month you enroll.

Step 3: Delay Part B and Part D (if covered)

If your employer plan qualifies, do nothing for Part B and Part D during your IEP. Mark your calendar. You will need to act when your employment or coverage ends.

Step 4: Enroll during the SEP

When you retire or lose coverage:

• For Part B: You have 8 months from the month after your employment ends or coverage ends (whichever happens first) to enroll. Call Social Security (1-800-772-1213) or apply online at ssa.gov. Use form CMS-40B or complete the online Medicare enrollment.
• For Part D: You have 2 months from losing creditable drug coverage to enroll in a stand-alone Part D plan or a Medicare Advantage plan with drug coverage. Use Medicare.gov/plan-compare to shop.

Step 5: Verify your coverage start date

Your Part B coverage generally starts the month after you enroll. Part D coverage starts the first of the month after you join a plan. Plan ahead to avoid a gap — if you delay enrollment until the last month of the SEP, your coverage may not start until the following month.

What If You Miss the SEP?

If you miss the Part B SEP, you’ll have to wait for the General Enrollment Period (January 1–March 31 each year). Your coverage starts July 1, and the late enrollment penalty is permanent — 10% of the standard premium per 12-month delay, added for life. For Part D, a missed SEP triggers a penalty of 1% of the national base beneficiary premium per month delayed, also permanent.

Costs and Penalties to Know

Item	2025 Value	Notes
Part B standard monthly premium	$185.00	Before any late penalty or IRMAA surcharge
Part B late enrollment penalty	10% per full 12-month delay	Added permanently to your Part B premium
Part B annual deductible	$257	Per year
Part D late enrollment penalty	1% of national base beneficiary premium ($36.78 in 2025) per month delayed	Added permanently to your Part D premium
Part D coverage phases	Deductible, initial, coverage gap, catastrophic	Varies by plan; standard deductible max is $590 in 2025
IRMAA surcharge (higher income)	Income > $106,000 (individual) or > $212,000 (married filing jointly)	Additional premium added to Part B and Part D; file SSA-44 to appeal if work status changes

Important: IRMAA (Income Related Monthly Adjustment Amount) applies if your modified adjusted gross income is above those thresholds. If you retire and your income drops significantly, you can request a reduction using form SSA-44. Keep a copy of your retirement date letter as evidence.

Expert Tips for a Smooth Enrollment

Tip 1: Get written proof of coverage now

Actionable step: Request a “Creditable Coverage Notice” from your employer for both medical and drug coverage. If you lose the letter later, you can use pay stubs or benefit summaries, but a formal letter is best.

Common mistake: Assuming “creditable” means any insurance. If your employer’s drug plan is not creditable, you can still delay Part B, but you must sign up for Part D separately at 65 or face a penalty. Verify every year — employers must provide the notice annually.

Tip 2: Check if your spouse’s plan covers you

Actionable step: If you are covered as a dependent on your spouse’s employer plan, the same 20-employee rule applies to their employer. Ask their HR for group size and creditable coverage.

Common mistake: Assuming your own employer coverage qualifies but not checking your spouse’s employer size. If your spouse’s employer has fewer than 20 employees, you must enroll in Part B and Part D at 65 regardless of their plan.

Tip 3: Enroll in Part B before the SEP ends, even if you still have other coverage

Actionable step: Mark the SEP deadline on your calendar. For Part B, it’s 8 months after you stop working. Don’t wait until the last month — processing delays can push your coverage start date past the deadline.

Common mistake: Thinking COBRA or retiree insurance extends the SEP. It does not. Once you leave the job, the clock starts. If you sign up after the 8-month window, you trigger the late penalty.

Quick Decision Aid: Is Delaying Medicare Right for You?

Use this checklist to confirm your situation. Each item should be a clear Yes or No.

1. [ ] Your employer (or your spouse’s employer) has 20 or more employees.

• If No, you cannot delay Part B — enroll at 65.

2. [ ] Your current health plan is creditable for Part D (drug coverage).

• If No, enroll in a standalone Part D plan at 65 to avoid the drug penalty.

3. [ ] You are not covered by a retiree insurance plan or COBRA as your primary coverage.

• If No, the SEP rules do not apply — you must use your IEP.

4. [ ] You understand that enrolling in Part A will stop HSA contributions but carries no penalty.

• If you want to keep HSA contributions, delay Part A too — but be aware that Part A is free for most people.

5. [ ] You have obtained written documentation from your employer confirming group size and creditable coverage.

• Pro tip: Store this letter with your other Medicare documents.

If all answers are Yes, you can safely delay Part B and Part D. If any No, adjust your enrollment plan accordingly.

Disclaimer: Medicare rules, premiums, and penalties change every year. The figures above (2025 Part B premium $185.00, IRMAA brackets, etc.) are based on current CMS data. Always verify at Medicare.gov or call 1-800-MEDICARE before making enrollment decisions. This article does not provide financial, legal, or medical advice. For personalized help, contact your State Health Insurance Assistance Program (SHIP).