Miss This 8-Month Window at 65 and Medicare Penalizes You Every Month for Life.

A 65-year-old leaves her job in March, signs up for COBRA to keep her doctors, and plans to deal with Medicare “later.” In November, an HR rep mentions the 8-month Special Enrollment Period. She does the math: her clock started when her employment or employer group coverage ended, not when COBRA ends. The window has been closing the whole time, and if she misses it, the Part B late enrollment penalty can follow her for as long as she has Medicare.

This article is for one specific reader: someone who is about to leave, or just left, active employer coverage at or after age 65. If you are still actively working and covered by an employer group health plan based on current employment, the clock may not have started. If you missed your Initial Enrollment Period and never had qualifying employer coverage, you are in a different situation than the one here.

What the 8-month window actually is

The Part B Special Enrollment Period gives you 8 months to sign up for Medicare Part B without a late enrollment penalty after employment or employer group health coverage ends, whichever happens first. The phrase that trips people up is employer coverage. For this SEP, the coverage must be tied to current employment. Once the job ends, COBRA or retiree coverage does not keep the Part B SEP open.

COBRA and retiree coverage do not extend the SEP. If you retire in March and your employer group health coverage also ends in March, your 8-month SEP starts the first full month after that, not when COBRA runs out. By the time 18 months of COBRA ends, you may be well past the deadline and forced to use another enrollment route, potentially with a late enrollment penalty.

The penalty math

Miss the window and Medicare can add 10% to your Part B premium for every full 12-month period you could have had Part B and did not. The surcharge generally lasts for as long as you have Part B. It attaches to whatever the standard premium is that year, so it rises when the standard premium rises.

The standard 2026 Part B premium is $202.90 per month, up from $185.00 in 2025. Run the surcharge against that:

At a three-year delay, a 68-year-old who finally enrolls in 2026 would owe an extra $60.87 every month for as long as she has Part B. Held flat against today’s premium, that is roughly $14,609 over 20 years, and it could grow because the surcharge tracks the standard premium. The standard Part B premium rose from $185.00 in 2025 to $202.90 in 2026, a 9.7% increase, so a static projection can understate the lifetime cost.

The Part D late enrollment penalty can stack on top of this if you also delay creditable drug coverage. It is calculated as 1% of the national base beneficiary premium for each full uncovered month and generally lasts as long as you have Medicare drug coverage.

When the clock actually starts

This is the sentence to write on a sticky note: the 8-month SEP starts when you stop working or lose employer group health coverage, whichever happens first, and Medicare describes the period as ending 8 months after that event.

Three implications people miss:

• If your last day of work is March 15 and your employer plan runs through March 31, treat April 1 as the practical start of the 8-month countdown, not the date COBRA terminates 18 months later.
• If you reduce hours and lose plan eligibility while still technically employed, the loss of employer group health coverage can start the clock.
• Part A is premium-free for most people, but people with HSA-eligible coverage should not automatically enroll at 65 because Medicare enrollment ends HSA contribution eligibility. The decision under pressure here is Part B.

What to do this week

1. Pull your employment end date and group health plan termination date in writing from HR. The earlier of those two dates, not your retirement party, anchors the SEP.

2. If you are on or considering COBRA at 65 or older, enroll in Part B during the SEP unless you have another valid enrollment strategy confirmed by Social Security or Medicare. Use COBRA for dental, vision, or family coverage if useful, but do not assume it stops the Medicare clock.

3. If you already missed the window, ask Social Security or Medicare whether an exceptional-conditions SEP or equitable relief applies before assuming the penalty is final. Form CMS-10797 is used to apply for Medicare Part A or Part B during a Special Enrollment Period for exceptional circumstances, such as a natural disaster, loss of Medicaid, release from incarceration, or other circumstances beyond your control. Bad information from a federal employee may support equitable relief, but bad advice from an employer is not automatically enough.

COBRA Is Coverage. It Is Not a Pause Button.

COBRA can keep a doctor network in place after a job ends, but it does not keep the Medicare Part B clock from running. Before leaving work at 65 or later, retirees should get the employment and coverage end dates in writing, mark the 8-month SEP, and enroll before COBRA creates a false sense of safety.

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