Medicare is not strictly mandatory at 65, but parts of it are effectively automatic if you’re already receiving Social Security benefits. If you’re collecting Social Security when you turn 65, you’ll be enrolled in Part A (hospital coverage) and Part B (medical coverage) automatically. You can opt out of Part B, but declining Part A is more complicated and carries consequences. For people still working with employer insurance, there’s more flexibility, though the rules depend on the size of your employer.
Who Gets Enrolled Automatically
If you’re 65 or older and already receiving Social Security benefits, you’re automatically enrolled in Medicare Part A. You don’t need to do anything. Your Medicare card will arrive in the mail before your 65th birthday, and you’ll also be signed up for Part B unless you actively decline it.
People who haven’t yet claimed Social Security need to sign up for Medicare on their own. Your Initial Enrollment Period is a seven-month window that starts three months before the month you turn 65 and ends three months after it. Missing this window can mean gaps in coverage and financial penalties that follow you for years.
Can You Refuse Part A?
Technically, yes. But there’s a catch. If you’re receiving Social Security retirement benefits, turning down Part A means giving up those benefits and repaying everything you’ve already received. That makes declining Part A a non-starter for most people. The reason this rule exists is that Part A and Social Security are linked at a structural level: once you’re drawing retirement benefits, the government treats hospital coverage as part of the package.
For most people, Part A is also free. You qualify for premium-free Part A if you or your spouse paid Medicare taxes for at least 10 years (40 quarters). If you don’t meet that threshold, you can still buy Part A, but it costs either $311 or $565 per month depending on how many years of work history you have.
When You Can Delay Without Penalty
If you’re still working at 65 and covered by an employer group health plan, you can delay enrolling in Part B (and sometimes Part A) without penalty. The key detail is the size of your employer. If the company has 20 or more employees, your employer plan is considered “primary” coverage, meaning it pays first and Medicare is secondary. In that case, you’re safe to wait.
If your employer has fewer than 20 employees, Medicare becomes the primary payer. That changes the math significantly. Your employer plan would only cover what Medicare doesn’t, and delaying Medicare could leave you underinsured. In companies with fewer than 20 workers, the normal Medicare Secondary Payer rules don’t apply, so Medicare is expected to be your main coverage.
Once you leave your job or lose that employer coverage, you get a Special Enrollment Period: two full months after the month your coverage ends to sign up for Medicare Part B. As long as you enroll within that window, no penalty applies.
Late Enrollment Penalties
The penalties for missing your enrollment windows are permanent, which is why this decision matters so much.
For Part B, you’ll pay an extra 10% on your monthly premium for every full 12-month period you could have been enrolled but weren’t. That surcharge stays on your premium for as long as you have Part B. If you waited three years without qualifying coverage, your premium would be 30% higher for the rest of your life.
For Part D (prescription drug coverage), the penalty is 1% of the “national base beneficiary premium” for each month you went without creditable drug coverage. In 2026, that base premium is $38.99. So if you went 24 months without creditable coverage, you’d owe roughly an extra $9.36 per month on top of whatever your Part D plan charges. This penalty also lasts indefinitely and adjusts as the base premium changes each year.
Creditable drug coverage means any prescription plan that pays, on average, at least as much as Medicare’s drug benefit. Employer plans, union plans, TRICARE, and VA coverage all count. Discount cards, free clinics, and drug manufacturer programs do not.
The HSA Complication
If you have a Health Savings Account through your employer, Medicare enrollment creates a tax issue you need to plan around. You cannot contribute to an HSA once you’re enrolled in any part of Medicare. That alone isn’t surprising, but here’s the part that catches people off guard: when you apply for Medicare Part A after age 65, your coverage is backdated up to six months (or to the month you turned 65, whichever is closer).
That retroactive coverage means any HSA contributions you made during those backdated months become excess contributions, which trigger tax penalties. The standard advice is to stop HSA contributions at least six months before you plan to apply for Medicare. If you’re turning 65 and want to keep contributing to your HSA, you’ll need to delay your Medicare application accordingly, which is only an option if you have qualifying employer coverage.
Part B Is Optional but Costly to Skip
You can decline Part B when you’re auto-enrolled. Part B covers doctor visits, outpatient care, preventive services, and medical equipment. If you have solid employer coverage, declining Part B temporarily makes sense. But if you don’t have other qualifying coverage, skipping Part B leaves you without outpatient medical insurance and starts the clock on late penalties.
The General Enrollment Period runs from January 1 through March 31 each year, with coverage starting July 1. That’s your fallback if you miss your Initial Enrollment Period and don’t qualify for a Special Enrollment Period. But enrolling this way means you’ll face both a gap in coverage and the permanent premium surcharge.
The bottom line: Medicare isn’t legally compulsory at 65 for everyone, but the system is designed to make enrollment the default. Between automatic enrollment for Social Security recipients, permanent late penalties, and the loss of HSA eligibility, most people are better off enrolling on time unless they have a clear reason to delay, backed by qualifying employer coverage from a company with 20 or more employees.