Turning 65 while still on the payroll creates one of the most misunderstood situations in US health coverage. Most people assume Medicare automatically takes over at 65, but federal coordination-of-benefits rules say otherwise. Workers at employers with 20 or more employees keep their employer plan as the primary payer, and Medicare steps back as secondary or can be delayed entirely for Part B. Getting this wrong in either direction is expensive. Taking Part B unnecessarily costs $202.90 per month in 2026 on top of your employer premium. Skipping Part B enrollment when you should not means a 10 percent per year lifetime penalty that compounds with every passing year. The decision hinges on three variables: your employer's size, whether your employer coverage is considered creditable for each Medicare part, and whether you contribute to a Health Savings Account (HSA). This guide walks through all three variables in 2026 terms, with the exact deadlines and documents you need at each decision point.
Federal law under Section 1862(b) of the Social Security Act sets the coordination rules. Employers with 20 or more employees must offer the same group health plan to workers age 65 and older that they offer to younger workers, and that plan pays first. For workers at smaller employers (under 20 employees), the coordination flips: Medicare becomes the primary payer at 65, and the small-group plan wraps around it. Many small-group plans actively refuse to pay claims that Medicare should have covered first, effectively leaving a coverage gap if you did not enroll in Medicare on time. Check your employer's size by counting all employees across all locations in the US, not just your office. Part-time and full-time employees both count for the 20-employee threshold. Beyond the employer-size question, the HSA interaction matters: enrolling in any part of Medicare, including premium-free Part A, stops your ability to contribute to an HSA going forward. Workers who plan to continue contributing to an HSA past 65 must delay both Part A and Part B, which requires careful coordination with your HR department and a written confirmation from your insurer that your plan is HSA-eligible. For details on the Medicare eligibility rules that apply at the federal level, see the Medicare eligibility guide. For the intersection of Medicaid and Medicare for lower-income seniors, check the Medicaid income limits page.
7 Steps to Get Coverage
Common Mistakes That Cost People Thousands
The most costly mistakes workers make when turning 65 while still employed. Most of these consequences are permanent:
- Assuming COBRA or retiree coverage counts for delaying Part B. Federal rules say only active group coverage from a current employer with 20 or more employees allows penalty-free Part B delay. COBRA and retiree health plans do NOT qualify. Take Part B at 65 if you go on COBRA or are on a retiree plan, or face the 10 percent per year lifetime penalty.
- Enrolling in Part A while still contributing to an HSA. Any Medicare enrollment, including premium-free Part A, makes you ineligible for HSA contributions. If you want to continue HSA contributions at 65, delay both Part A and Part B and confirm in writing that your plan is HSA-eligible. The 2026 HSA limit is $4,400 for self-only coverage and $8,750 for family coverage.
- Missing the 8-month Part B SEP after employer coverage ends. Workers who successfully delay Part B must enroll within 8 months of coverage ending. Many retirees assume they have 60 days like other SEPs, but the employer-retirement SEP is 8 months. Missing it means waiting for the General Enrollment Period (January 1 through March 31 each year) with coverage starting July 1, creating a coverage gap and triggering the lifetime penalty.
- Not getting a creditable-coverage notice from HR before October 15 each year. CMS requires employers to notify Medicare-eligible employees annually whether their drug plan is creditable. If you cannot show you had creditable drug coverage during any gap, CMS assesses the Part D late penalty for that entire period.
- Missing the 6-month Medigap guaranteed-issue window after Part B starts. Once you retire and take Part B, you have exactly 6 months to buy any Medigap plan without medical underwriting. After that window closes, insurers in most states can refuse to sell you a Medigap policy or charge more because of health history, often permanently pricing you out.
The 20-Employee Rule and Coordination of Benefits in 2026
Federal coordination-of-benefits rules under Section 1862(b) of the Social Security Act split Medicare-eligible workers into two groups based purely on employer size. Workers at employers with 20 or more employees fall under the Working Aged provision: the employer plan pays primary, Medicare pays secondary or can be fully delayed. Workers at employers with fewer than 20 employees face Medicare Secondary Payer rules in reverse: Medicare becomes the primary payer at 65, and the small-group employer plan wraps around it. Many small-group insurers write explicit exclusions into their plans that deny payment for claims Medicare should have covered, creating a coverage gap that looks identical to having no insurance at all for those services. Confirming your employer's total US headcount is the single most important first step for any worker approaching 65.
Employer size counts all employees in the US, including part-time workers, across all locations and subsidiaries that form a single entity. A company with 18 full-time employees and 5 part-time employees counts as 23 employees, clearing the 20-employee threshold. Seasonal workers are typically counted differently, so verify directly with HR rather than estimating. Once you confirm employer size, request a letter from HR confirming that your plan is an active group health plan and that you are a currently active employee. Keep that letter for your records because you will need it when you eventually apply for Part B under the employer-retirement SEP.
HSA Contributions and Medicare: The Often-Missed Conflict
Health Savings Accounts (HSAs) offer powerful triple-tax-free benefits for workers on high-deductible health plans: contributions are pre-tax, growth is tax-free, and qualified withdrawals are tax-free. However, federal law under IRC Section 223 prohibits HSA contributions once you are enrolled in any part of Medicare, including premium-free Part A. For 2026, the HSA contribution limit is $4,400 for self-only coverage and $8,750 for family coverage, per IRS Rev. Proc. 2025-19. Workers who want to continue contributing past age 65 must delay both Part A and Part B as long as they remain on an HSA-eligible high-deductible health plan through active employment.
A critical 6-month lookback rule applies to Part A: Social Security retroactively assigns Part A coverage to the 6 months before your application date, up to your 65th birthday. Workers who delay Part A must stop HSA contributions at least 6 months before they plan to apply for Social Security or Medicare, or face an IRS penalty on excess HSA contributions for those retroactive months. If you plan to apply for Social Security at age 70 to maximize your benefit, stop HSA contributions no later than 6 months before that application date. Consult a benefits advisor or your tax preparer about the exact timing. The IRS penalty for excess HSA contributions is 6 percent per year, and the excess must be withdrawn with earnings, creating a taxable event.
Documents You Need for Medicare Enrollment After Employer Coverage Ends
Enrolling in Part B under the employer-retirement Special Enrollment Period requires documentation that proves your delay was legitimate. The Social Security Administration uses Form CMS-L564 (Request for Employment Information) filled out by your employer to confirm your active coverage dates. Missing or incomplete documentation is the most common reason Part B SEP applications are delayed or denied. Gather these documents before your last day of work to avoid a coverage gap.
- Form CMS-L564 completed by your employer (confirms active employment and group health plan dates)
- Annual Notice of Creditable Coverage for drug plan (from HR, received before October 15 each year)
- Medicare card (Parts A and B) if you enrolled in Part A earlier
- Proof of prior HSA-eligible plan if delaying both A and B (plan summary documents showing HDHP status)
- Social Security number and photo ID (driver's license or passport) for SSA.gov enrollment
- COBRA election notice (if applicable) to establish timeline of active coverage vs. COBRA start
Medicare Costs in 2026 for Workers Who Enroll at Retirement
Standard Medicare costs for new enrollees in 2026 are set by CMS. Part B premium is $202.90 per month for individuals with modified adjusted gross income (MAGI) at or below $109,000 (single filers) or $218,000 (joint filers). Higher earners pay more through IRMAA surcharges, which are based on your income from 2 years prior, meaning your 2024 income determines your 2026 IRMAA bracket. Workers transitioning from high-salary jobs to retirement should expect elevated IRMAA for up to 2 years before income drops reflect in their premiums. File Form SSA-44 (Medicare Income-Related Monthly Adjustment Amount Life-Changing Event) with Social Security to request a reduction based on retirement income if your income dropped significantly.
Part A in 2026 carries a $1,736 per-benefit-period hospital deductible (not an annual deductible; it resets with each separate hospital stay after 60 days). Part B has a $283 annual deductible, then 20 percent coinsurance for most services with no out-of-pocket cap under Original Medicare alone. That absence of a cap is the primary reason workers choose either Medicare Advantage (which has the $9,250 in-network MOOP ceiling for 2026) or Medigap (which fills the 20 percent coinsurance). The 2026 Part D out-of-pocket cap is $2,100, set permanently by the Inflation Reduction Act of 2022 for all Medicare Part D enrollees.
Frequently Asked Questions
Can I delay Medicare Part B if I am still working at 65?
Yes, if you have active group health coverage through your own or your spouse's employer and that employer has 20 or more employees. Under that condition, your employer plan pays primary and you can delay Part B without a penalty using the employer-retirement Special Enrollment Period (SEP). You get 8 months to enroll in Part B after your active coverage ends. Workers at employers with fewer than 20 employees cannot delay Part B without penalty because Medicare becomes their primary payer at 65.
What is the Medicare enrollment window when I turn 65 and am still working?
Every worker turning 65 has a 7-month Initial Enrollment Period (IEP): 3 months before the birthday month, the birthday month, and 3 months after. Workers with large-employer coverage can decline Part B during the IEP and later use the 8-month employer-retirement SEP when they stop working. The IEP window for someone turning 65 in August 2026 runs from May 1, 2026 through November 30, 2026. Workers at small employers (under 20 employees) should enroll in both Part A and Part B during the IEP to avoid the lifetime penalty.
What documents do I need to prove I legitimately delayed Part B?
When you apply for Part B under the employer-retirement SEP, the Social Security Administration requires Form CMS-L564 (Request for Employment Information) completed and signed by your employer. This form confirms the dates your active group health plan coverage was in effect. You should also keep your annual Notice of Creditable Coverage from HR for the drug plan, and any summary plan documents showing your plan was active employer-sponsored coverage. Without Form CMS-L564, your SEP application may be delayed or denied, triggering the General Enrollment Period process and a coverage gap.
Does COBRA count as employer coverage for delaying Part B without penalty?
No. COBRA continuation coverage does not count as active group health coverage under the Medicare Secondary Payer rules. Only active coverage from a current employer (your own or your spouse's) with 20 or more employees qualifies for the Part B delay exemption. If you go on COBRA after losing your job at age 65, you must enroll in Part B within the remaining time in your IEP or during the 60-day COBRA qualifying event window. Failing to do so triggers the 10 percent per year lifetime penalty. Retiree health plans from former employers also do not count.
How does having an HSA affect my Medicare enrollment decision at 65?
Enrolling in any part of Medicare, including premium-free Part A, immediately disqualifies you from making further HSA contributions under IRC Section 223. For 2026, the annual HSA limit is $4,400 for self-only coverage and $8,750 for family coverage. Workers who want to keep contributing must delay both Part A and Part B by remaining on an HSA-eligible high-deductible health plan through active employment. Critically, Social Security applies a 6-month retroactive Part A rule when you eventually apply, so stop HSA contributions 6 months before your planned Medicare start date to avoid IRS penalties on excess contributions.
What happens if I miss the 8-month Part B SEP after retiring?
Missing the 8-month employer-retirement SEP means you must wait for Medicare's General Enrollment Period, which runs January 1 through March 31 each year, with coverage starting July 1 of that year. This can create a coverage gap of 3 to 18 months depending on when you retired. During that gap, an ACA Marketplace plan may bridge coverage if you have no other source, though Marketplace SEP eligibility for Medicare-eligible individuals is limited. You will also owe the 10 percent per year Part B lifetime penalty for every full 12-month period you were eligible but not enrolled, starting from when your IEP ended. That penalty is added permanently to every Part B premium you ever pay.
What is the Part B late enrollment penalty in 2026?
The Part B late enrollment penalty is 10 percent added to your monthly Part B premium for each full 12-month period you could have had Part B but did not enroll, and you had no valid exemption (such as active employer coverage). The penalty is permanent and lasts the rest of your life. At the 2026 standard Part B premium of $202.90, two 12-month periods of non-enrollment would add roughly $40.58 per month forever. Higher-income enrollees paying IRMAA surcharges see the 10 percent applied to their base premium before the IRMAA add-on.
Should I enroll in Part A at 65 even if I am not taking Part B yet?
Most workers should enroll in premium-free Part A at 65 even while delaying Part B, because Part A costs nothing for those with 10 or more years of Medicare-covered work. Part A adds hospital coverage as secondary to your employer plan. The exception: if you actively contribute to an HSA, enrolling in Part A triggers the HSA contribution prohibition. Workers contributing to an HSA should delay both Part A and Part B until they stop contributing. Ask HR to confirm in writing that your plan is HSA-eligible before making this decision.
