Signing up for Medicare is not legally mandatory, but the system is designed so that most people at 65 are enrolled automatically, and opting out carries real financial consequences. If you’re already receiving Social Security benefits, you’ll be enrolled in Medicare Part A (hospital coverage) and Part B (medical coverage) without lifting a finger. You can decline Part B, but walking away from Part A entirely requires giving up Social Security and repaying every dollar you’ve received.
So while no law forces you to have Medicare, the practical reality is that most Americans end up enrolled. The real question is which parts you need, when you need to act, and what happens if you delay.
How Automatic Enrollment Works
If you’re already collecting Social Security or Railroad Retirement Board benefits at least four months before your 65th birthday, Medicare enrolls you in both Part A and Part B automatically. You don’t file a separate application. Your Medicare card arrives in the mail about three months before you turn 65.
You can choose to refuse Part B, which carries a monthly premium ($185 in 2025 for most people). But premium-free Part A is a different story. Federal rules do not allow you to voluntarily terminate Part A coverage once you’re entitled to it through Social Security. The only way to drop Part A is to withdraw from Social Security entirely and repay every benefit payment you’ve received. For someone who has collected even a year or two of Social Security checks, that repayment can easily reach tens of thousands of dollars.
When You Do Need to Sign Up Yourself
If you haven’t started collecting Social Security by the time you turn 65, Medicare won’t enroll you automatically. You’ll need to sign up during your Initial Enrollment Period: a seven-month window that starts three months before your 65th birthday and ends three months after the month you turn 65. You can enroll through Social Security’s website or your local office, choosing Part A only or both Part A and Part B.
Missing that window doesn’t mean you’re locked out forever, but it does trigger penalties that follow you for the rest of your time on Medicare.
Late Enrollment Penalties
The biggest practical reason to sign up on time is the Part B late enrollment penalty. For every full 12-month period you could have had Part B but didn’t, your monthly premium increases by 10%. That surcharge is permanent. It gets added to your Part B premium for as long as you have Medicare.
Part D (prescription drug coverage) has its own penalty. If you go 63 or more consecutive days without drug coverage that’s at least as comprehensive as Medicare’s, you’ll pay an extra 1% of the national base beneficiary premium for every uncovered month. In 2026, the base premium is $38.99, so someone who waited 14 months would pay an extra $5.50 per month on top of their drug plan premium, and that penalty also lasts indefinitely.
Delaying Medicare With Employer Coverage
There is one clean path to delay enrollment without penalties. If you or your spouse are still working and covered by a group health plan from an employer with 20 or more employees, you can postpone Part B (and Part D) without consequence. The employer plan pays first, Medicare pays second, and when you eventually leave that job or lose that coverage, you’ll get a Special Enrollment Period to sign up penalty-free.
The 20-employee threshold matters. If the employer has fewer than 20 workers, Medicare becomes the primary payer even if you have employer coverage, which means you really need to enroll at 65 to avoid gaps in how your bills get paid. In that scenario, your employer plan only picks up what Medicare doesn’t cover.
Military Retirees and TRICARE
For military retirees, Medicare enrollment is effectively mandatory if you want to keep TRICARE for Life. Once you become Medicare-eligible, TRICARE requires you to have both Part A and Part B to remain in the program, including its prescription drug benefit. Skipping Part B means losing TRICARE coverage entirely, not just supplemental benefits.
The HSA Complication
If you’re still working past 65 and contributing to a Health Savings Account through a high-deductible health plan, Medicare enrollment creates a tax problem. Once you enroll in any part of Medicare, you’re no longer eligible to contribute to an HSA. That’s straightforward enough, but there’s a lesser-known wrinkle: when you enroll in Medicare after 65, Part A coverage is applied retroactively for up to six months (though not before your 65th birthday).
That retroactive coverage means any HSA contributions you made during those six months are considered excess contributions by the IRS, which triggers tax penalties. The fix is to stop contributing to your HSA at least six months before you plan to enroll in Medicare. If you’ve already overcontributed, you can typically reverse the excess through your HSA administrator before you file taxes for that year. Otherwise, you’ll need to file an amended return.
This is particularly important to plan around if you’re delaying Social Security. Claiming Social Security after 65 triggers automatic Part A enrollment, which in turn triggers the six-month lookback. People who intend to keep funding an HSA past 65 need to time their Social Security application carefully.
Who Might Choose Not to Enroll
A small number of people genuinely don’t need or want Medicare. Some are covered by a working spouse’s large-employer plan. Others have religious objections or belong to healthcare sharing ministries. A few high-income individuals prefer to self-pay or carry only private insurance.
For most of these situations, the practical move is to accept premium-free Part A (which costs nothing and serves as a backstop for hospital stays) while declining Part B until you actually need it. Turning down free Part A only makes sense if you’re contributing to an HSA and want to avoid the retroactivity issue, or if you have a specific reason to stay completely outside the Medicare system. Even then, the penalties for delayed Part B enrollment and the requirement to repay Social Security benefits make full refusal a significant financial decision.