Still on Your Employer’s Plan at 65? The Medicare Enrollment Mistake That Costs $4,000 a Year

Carol turned 65, kept her employer health plan, skipped Medicare enrollment because she already had coverage, and when she finally enrolled at 68, she discovered she owed a 30% permanent premium surcharge that would follow her for the rest of her life.

Carol worked for a company with 12 employees. The health plan was good, and when she turned 65, her HR contact told her she could keep the plan and didn’t need to do anything about Medicare. That advice was incomplete in a way that cost Carol thousands of dollars. Because her employer had fewer than 20 employees, the federal Medicare Secondary Payer law made Medicare the primary payer for her claims at 65, whether she enrolled or not. Her employer’s insurer processed her claims as secondary to Medicare and then denied the portion Medicare was supposed to cover because she wasn’t enrolled. When she finally enrolled at 68 after leaving her job, she learned about the Part B late-enrollment penalty: 10% added permanently to her monthly premium for each full year she was eligible and didn’t enroll. At three years, that’s a 30% surcharge. At the 2026 standard Part B premium of $202.90 a month, that adds roughly $61 a month to her bill. Over a 20-year retirement, the total penalty approaches $14,600. She didn’t know she was making a decision at all.

The size of your employer determines whether delaying Medicare is smart planning or a permanent financial mistake. Most people never check the number.

The Rule That Changes Everything: Employer Size

The entire Medicare-plus-employer-coverage decision turns on a threshold that almost no one outside the HR and benefits world knows to look for: whether your employer has 20 or more employees.

For employers with 20 or more employees, the employer health plan is the primary payer for active employees aged 65 and over. Medicare is secondary. In that situation, delaying Medicare Part B enrollment while you’re still working and covered by an active employer plan is a legitimate strategy. You avoid paying two premiums, and you get a protected window after you leave employment to enroll in Medicare without facing a penalty. That window is called the Special Enrollment Period, and it lasts for eight months after you lose the qualifying employer coverage.

For employers with fewer than 20 employees, the rules are reversed. Federal Medicare Secondary Payer law designates Medicare as the primary payer for employees who are 65 and enrolled. The employer plan becomes secondary. If you’re not enrolled in Medicare at 65 and you’re on a small employer’s plan, the insurer processes your claims assuming Medicare will cover its portion first, then denies whatever Medicare was supposed to pay because you have no Medicare coverage. You end up with large unpaid portions of your medical bills and no clear path to recover them retroactively.

What Happens on a Large Employer Plan

If your employer has 20 or more employees and you’re still actively working at 65, you have options. The employer plan covers you as primary. You can delay Part B enrollment without penalty as long as you stay on that qualifying active coverage. The key phrase is “active” coverage from your own (or a spouse’s) current employment. Not COBRA, not retiree coverage, not coverage from a former employer’s plan.

When that coverage ends, the eight-month Special Enrollment Period begins. This is when you enroll in Medicare Part B without a late enrollment penalty. The most common mistake here is waiting too long. People who leave employment in March but don’t file Medicare paperwork until November, or who assume they need to wait for an open enrollment window, may miss the SEP and face a delayed entry through the General Enrollment Period (January through March each year), with coverage starting July 1 and a permanent penalty attached.

The other mistake is treating COBRA as qualifying coverage that allows you to delay Medicare. COBRA does not qualify as current employer coverage for purposes of the Special Enrollment Period. Using COBRA as a reason to delay Medicare enrollment will result in a penalty.

What Happens on a Small Employer Plan

If your employer has fewer than 20 employees, the calculus is completely different, and the stakes are higher. Medicare is primary the moment you turn 65, by federal law, regardless of whether you’ve enrolled. Your employer’s insurer knows this and builds it into its claims processing.

If you’re not enrolled in Medicare at 65, what happens in practice is that your employer’s insurer may deny claims outright, pay only a secondary-level amount, or process them inconsistently, depending on the insurer. Some small employer plans do not make this clear to employees. HR departments at small companies frequently don’t know the rule, or know it only vaguely. The result is that employees like Carol go months or years without a health plan that actually covers what they believe it covers.

Enrolling in Medicare Part A and Part B at 65 when you’re on a small employer plan isn’t optional in any practical sense. It’s the only way to ensure your healthcare claims are actually processed and paid.

The Late Enrollment Penalty: Permanent and Compounding

If you miss your Initial Enrollment Period (the seven-month window around your 65th birthday) without a qualifying reason to delay, you face the Part B late enrollment penalty. The penalty is 10% for each full 12-month period during which you were eligible for Part B but didn’t enroll. It is added permanently to your monthly Part B premium for as long as you have Part B coverage.

At the 2026 standard premium of $202.90 a month, a two-year delay adds $40 a month. A three-year delay adds $61 a month. A five-year delay adds $101 a month. These numbers don’t drop after you’ve paid for a few years. They don’t reset when circumstances change. They stay with you.

The Part B late enrollment penalty doesn’t have an expiration date. Pay it at 66, pay it at 86.

Questions to Ask Your HR Department and Your Benefits Advisor Before You Turn 65

• Does my employer have 20 or more employees for purposes of the Medicare Secondary Payer law?
• If I stay on the employer plan past 65 and my employer has fewer than 20 employees, how will my claims be processed?
• When my employer coverage ends, what is the exact start date of my Special Enrollment Period, and how long do I have?
• Does COBRA coverage qualify me to delay Medicare enrollment without a penalty?
• If I enroll in Medicare while still on an employer plan, how does coordination of benefits work for my specific plan?

The Part B late enrollment penalty is one of the few financial penalties in health insurance that is genuinely permanent. The decisions you make in the months around your 65th birthday cannot be undone retroactively. Getting specific answers from your HR department and a Medicare counselor before that window opens is the only way to avoid paying for a mistake for the next 20 years.

For a detailed breakdown of what Medicare covers in each part, see What Does Medicare Cover: A Simple Guide for Beginners. If you’re weighing Medicare Advantage against a Medigap supplement once you do enroll, Medicare Advantage vs. Medicare Supplement: Key Differences Explained walks through the tradeoffs.