A 64-year-old engineer takes a buyout from a 3,000-person employer in March. He signs up for 18 months of COBRA the same week, figuring he will deal with Medicare when COBRA runs out. Eleven months later, he turns 65 and walks into Social Security to enroll in Part B. The clerk pulls up his record, runs a quick calculation, and tells him his late enrollment penalty will follow him for the rest of his life.
He did not miss a deadline he knew about. He missed a deadline he did not know existed.
COBRA continues your plan. It does not extend your Medicare window.
Medicare’s 8-month Special Enrollment Period is the safety net that lets people delay Part B past 65 without penalty while they are still working. The catch most people never hear: the clock starts when the active-employment coverage ends, not when COBRA ends. COBRA keeps the same plan, the same network, often the same negotiated premium share. It does not extend the SEP by a single day. Neither does retiree health coverage.
The Special Enrollment Period only applies when the beneficiary, or their spouse, is currently working for an employer with 20 or more employees and covered through that active job. The moment the paycheck stops, the 8-month timer begins.
For the engineer above, who left active employment in March and turned 65 the following February, the clock had already been running for 11 months by the time he tried to enroll. Because he became eligible for Medicare during that period, his Special Enrollment Period was tied to the end of his active-employment coverage, not the end of COBRA. By the time he sat down at Social Security, his SEP had already expired.
The penalty math, in dollars he can check
Part B’s late enrollment penalty is generally 10% for each full 12-month period a person could have had Part B but did not enroll. The penalty is typically permanent. It attaches to future Part B premiums and rises as the underlying premium rises.
The 2026 standard Part B premium is $202.90 a month, up from $185.00 in 2025. A 10% penalty adds about $20.29 a month, or roughly $243.48 a year, on top of the standard premium. Held flat at today’s premium as a conservative floor, a single 12-month delay would cost roughly $4,870 over 20 years. Because Medicare premiums tend to rise over time, the actual lifetime cost could be higher.
Two full years past the SEP doubles the rate. That is about $487 a year in extra premium, forever, at today’s price.
Why so many people walk into this
COBRA feels like a continuation of employer coverage, because legally it is one. The election packet arrives in the same envelope as the severance paperwork, and nothing in it flags that Medicare treats it as a different animal. Part D’s creditable-coverage rules deepen the confusion: some retiree drug plans and VA prescription coverage do count as creditable for the separate Part D penalty, which leads people to assume Part B works the same way. It does not.
Two more details worth getting right before you make a decision:
- The fall Annual Enrollment Period, October 15 to December 7, is for Medicare Advantage and Part D changes. It is not a Part B enrollment window for people who missed their SEP. They have to use the General Enrollment Period, January 1 through March 31, with coverage starting the month after they sign up.
- Employers with fewer than 20 employees follow different rules. Medicare is generally primary at 65, and staying on the small-employer plan without enrolling in Part B can create coverage gaps and penalties regardless of COBRA status.
What to do
If you are approaching 65 and weighing COBRA, treat the day your job ends, or the day you turn 65 if you are already past employment, as the start of an 8-month Part B countdown. Enroll inside that window even if your COBRA premium feels comfortable. COBRA and Part B can run alongside each other for the overlap months. The SEP cannot be rewound once it closes.
If your SEP has already expired and you are still on COBRA, enroll during the next General Enrollment Period rather than waiting for COBRA to run out. Every additional 12-month block you sit out adds another 10% to the penalty, permanently.
If you believe a penalty was applied in error, request Form CMS-L564 (Request for Employment Information) from your former employer documenting your active coverage dates, and ask Social Security for a formal determination. The appeals path is narrow. Documented misinformation from a federal employee or a verifiable gap in the employer’s coverage record can sometimes reverse it.
The dollars look small on any single month’s premium. The compounding is what bites.
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