When you turn 65, Medicare becomes your primary health insurance in most situations, but your existing coverage doesn’t just vanish overnight. What actually happens depends on whether you’re still working, how large your employer is, and whether you’re on a spouse’s plan, COBRA, or an individual policy. The key date to know: your Initial Enrollment Period is a 7-month window that starts 3 months before your 65th birthday month and ends 3 months after it. Missing that window can cost you permanently higher premiums.
Your 7-Month Enrollment Window
Medicare gives you a specific period to sign up called the Initial Enrollment Period. It spans seven months total: the three months leading up to your birthday month, your birthday month itself, and the three months following. If you’re already receiving Social Security benefits, you’ll be automatically enrolled in Medicare Parts A and B. If you’re not collecting Social Security yet, you need to sign up yourself through the Social Security Administration.
Part A covers hospital stays and is premium-free for most people who paid Medicare taxes for at least 10 years. Part B covers doctor visits, outpatient care, and preventive services. In 2026, the standard Part B premium is $202.90 per month, though higher earners pay more based on income.
If You’re Still Working at 65
This is where employer size matters. If your employer has 20 or more employees, your employer plan stays primary and Medicare becomes secondary. You can delay enrolling in Part B without penalty as long as you have coverage through active employment (yours or your spouse’s). Once you stop working or lose that coverage, you get a Special Enrollment Period of 8 months to sign up for Part B penalty-free.
If your employer has fewer than 20 employees, the rules flip. Medicare becomes your primary payer the moment you’re eligible, and your employer plan pays second. In this situation, you should enroll in Medicare during your Initial Enrollment Period, because your employer plan will only cover what Medicare doesn’t. Skipping Medicare here means you’d be stuck paying most costs out of pocket, since your small-employer plan expects Medicare to handle the bulk of the bill.
What Happens to COBRA Coverage
COBRA and Medicare don’t mix well. If you’re on COBRA when you become eligible for Medicare, your COBRA coverage will likely end once you enroll. More importantly, if you’re eligible for Medicare but don’t sign up, COBRA may only pay a small portion of your health care costs, leaving you responsible for most of the bill. COBRA does not count as employer coverage for the purpose of delaying Medicare enrollment, so don’t treat it as a reason to wait. You have 8 months after you stop working or lose your employer insurance (whichever comes first) to enroll in Part B without a penalty, regardless of whether you elect COBRA.
The Cost of Enrolling Late
If you miss your enrollment window and don’t qualify for a Special Enrollment Period, you’ll pay higher premiums for the rest of the time you’re on Medicare. The Part B penalty adds 10% to your monthly premium for every full 12-month period you could have been enrolled but weren’t. So if you waited three years without qualifying coverage, your Part B premium would be 30% higher permanently.
Part D (prescription drug coverage) has its own penalty: 1% of the national base premium added for every month you went without creditable drug coverage. That adds up to 12% per year of delay. The penalty kicks in if you go 63 or more days without drug coverage that’s at least as good as Medicare’s. Coverage from an employer, union, TRICARE, or the VA typically qualifies as creditable. Discount cards, free clinics, and drug samples do not.
Your Health Savings Account Changes
If you have a Health Savings Account tied to a high-deductible health plan, Medicare enrollment changes the rules. You can no longer contribute to an HSA once you’re enrolled in any part of Medicare. The tricky part: when you sign up for Part A, Medicare provides up to six months of retroactive coverage going back to your eligibility date. That means if you keep contributing to your HSA right up until you enroll, some of those contributions may overlap with your Medicare coverage period and trigger a tax penalty. Stop contributing at least six months before you plan to enroll to stay clear.
You can still spend money already in your HSA on qualified medical expenses, including Medicare premiums, deductibles, and copays. You just can’t put new money in.
Choosing Between Original Medicare and Medicare Advantage
Once you’re enrolled, you have two main paths. Original Medicare (Parts A and B) lets you see any doctor or hospital in the country that accepts Medicare. There’s no network restriction, but there’s also no annual cap on your out-of-pocket spending unless you buy supplemental coverage. Most people on Original Medicare add a Medigap (Medicare Supplement) policy to help cover deductibles, copays, and coinsurance. You’d also need to join a separate Part D plan for prescription drugs.
Medicare Advantage plans (Part C) are offered by private insurers and bundle hospital, medical, and usually prescription drug coverage into one plan. They include an annual out-of-pocket maximum, which Original Medicare lacks. Once you hit that limit, the plan covers 100% of your costs for the rest of the year. The trade-off is that most Advantage plans use provider networks, meaning you may need to use specific doctors and hospitals for non-emergency care, and going out of network costs significantly more.
Special Enrollment Periods After 65
Life changes after 65 can open new enrollment windows. Losing employer or union coverage (including COBRA) gives you two full months after the month your coverage ends to join a Medicare drug plan or Advantage plan. Moving out of your plan’s service area gives you two months to switch. Returning to the U.S. after living abroad, leaving a nursing home or rehabilitation facility, or being released from incarceration all trigger their own Special Enrollment Periods, each lasting about two months.
If you have both Medicare and Medicaid, or if you qualify for Extra Help paying for prescriptions, you can change your coverage once per calendar month, with the new plan taking effect the first day of the following month.