Marketplace plans and Medicare cover different populations. Marketplace is for people under 65 (mostly) who do not have other coverage. Medicare is for people 65 or older and certain younger people with disabilities or end-stage renal disease.
The two programs do not stack. Once you are eligible for Medicare, most marketplace doors close.
When you can stay on a marketplace plan
You can keep a marketplace plan past 65 in narrow cases:
You decline to enroll in Medicare. You can do this, but you usually pay late enrollment penalties later that last for life, and you lose the marketplace premium tax credit the month you become eligible for premium-free Part A.
You are 65 or older but not yet eligible for premium-free Part A. This is rare. Most people qualify for premium-free Part A based on their own or a spouse work history.
For nearly everyone turning 65, the answer is: you move to Medicare. Marketplace is a bridge until then.
The basic parts of Medicare
Part A. Hospital insurance. Covers inpatient hospital stays, skilled nursing facility, hospice, some home health. Most people pay no Part A premium because they paid Medicare taxes for at least 40 quarters (10 years) of work.
Part B. Medical insurance. Covers doctor visits, outpatient services, durable medical equipment, preventive care. Has a monthly premium set each year by CMS. Higher-income people pay more under IRMAA (Income-Related Monthly Adjustment Amount).
Part C (Medicare Advantage). A private alternative to Original Medicare. Bundles Parts A and B, usually includes Part D, and often adds extras like vision, dental, hearing, or fitness. Plans have networks and prior authorization. Plans are sold by private insurers.
Part D. Prescription drug coverage. Sold by private insurers. Has its own enrollment, formulary, and premium. Recent changes from the Inflation Reduction Act capped out-of-pocket prescription costs at a new annual ceiling and capped insulin costs. The cap and details change. Check Medicare.gov for current numbers.
Medigap (Medicare Supplement). Private policies that fill gaps in Original Medicare (Part A and B), such as the Part B 20 percent coinsurance. Sold under standardized plan letters (Plan G, Plan N, etc.) with the same benefits across insurers in most states.
The Initial Enrollment Period
The Initial Enrollment Period (IEP) is seven months around your 65th birthday: the three months before, your birth month, and the three months after.
For people getting Social Security or Railroad Retirement Board benefits before 65, enrollment in Part A and Part B is automatic.
For everyone else, you sign up through Social Security (ssa.gov/medicare/sign-up) during the IEP.
Coverage start dates depend on when you enroll within the IEP. In recent years the rule has been: coverage starts the month after you sign up, with limited exceptions. The exact rule has changed, so confirm on Medicare.gov.
If you miss the IEP and do not have a Special Enrollment Period:
General Enrollment Period runs January 1 to March 31, with coverage usually starting the month after you sign up.
Late enrollment penalties may apply, especially for Part B and Part D. The Part B penalty is 10 percent of the standard premium for each full 12-month period you could have been enrolled but were not, and it lasts as long as you have Medicare.
When employer coverage delays Medicare
If you are still working at 65 and have employer coverage:
Large employer (20 or more employees). The employer plan stays primary. You can delay Part B without penalty. When you eventually leave the employer plan, an eight-month Special Enrollment Period lets you sign up for Part B.
Small employer (fewer than 20 employees). Medicare typically becomes the primary payer at 65 even if you do not enroll. You usually should enroll in Part A and B at 65 to avoid coverage gaps.
COBRA does not count as active employer coverage for Medicare purposes. The SEP for Part B based on employer coverage ends when active employment ends, not when COBRA ends.
Medicare and HSAs
This catches a lot of people.
You cannot contribute to an HSA once you enroll in any part of Medicare, including premium-free Part A.
When you claim Social Security at or after 65, you are automatically enrolled in Part A, retroactive up to six months.
If you wanted to keep contributing to an HSA past 65, you need to delay both Social Security and Medicare. You can do this, but Social Security delayed retirement credits and Medicare timing have separate consequences. Plan with a tax pro or SHIP counselor.
Original Medicare vs Medicare Advantage
Original Medicare (Parts A and B, usually with a Part D plan and a Medigap).
Pros: any provider that accepts Medicare nationwide, no plan network, predictable rules.
Cons: no annual out-of-pocket maximum unless you add a Medigap policy, Part B coinsurance is 20 percent, separate Part D for drugs.
Medicare Advantage (Part C).
Pros: bundled coverage with extras (dental, vision, hearing, fitness benefits), usually with an out-of-pocket maximum, often $0 plan premium (but Part B premium still required).
Cons: network restrictions, prior authorization for many services, plans vary by ZIP code, can change benefits each year.
Both have legitimate use cases. The choice depends on whether you travel, whether your providers are in network, and how you weigh predictability against benefit richness.
The Annual Election Period in fall (October 15 to December 7) lets you switch plans for the following year. The Medicare Advantage Open Enrollment Period (January 1 to March 31) allows one more switch.
Common pitfalls
Missing the Part B sign-up. Late penalties last for life.
Assuming COBRA counts as employer coverage for Medicare. It does not.
Filing for Social Security at 65 while contributing to an HSA. The automatic Part A enrollment ends HSA contributions immediately and can require corrective distributions.
Comparing only premium when picking Medicare Advantage. The network and the drug formulary matter more.
Buying Medigap outside the Medigap Open Enrollment window. Outside that six-month window starting when you enroll in Part B at 65 or older, insurers can underwrite based on health.
What to do next
Six months before 65, start reading Medicare.gov and watch for your "Welcome to Medicare" mailing if you are getting Social Security.
Three months before 65, decide whether to enroll in Part B based on your employer situation.
If you are confused, call your State Health Insurance Assistance Program (SHIP). It is free, neutral counseling.
For broader context, see our ACA marketplace overview and open enrollment deadlines.
Sources
Frequently asked questions
Can I have both Medicare and a marketplace plan?
You generally should not. Once you are eligible for or enrolled in Medicare, marketplace premium tax credits stop. The marketplace will not knowingly sell you a plan as your primary coverage if you have Medicare Part A.
When should I enroll in Medicare?
For most people, during the seven-month Initial Enrollment Period that starts three months before you turn 65 and ends three months after.
What if I have employer coverage at 65?
If the employer has 20 or more employees, you can usually delay Medicare without penalty. Smaller employer coverage often becomes secondary to Medicare even if you do not enroll, so you usually should enroll.
Do HSA contributions stop at 65?
HSA contributions stop when you enroll in any part of Medicare, including Part A. People who file for Social Security at 65 are automatically enrolled in Part A and lose the ability to contribute.
